Choosing a convertible or FF harnessed seat
When you're looking for the safest convertible seat, you're really looking for two things:
1) Keeping your child rearfacing as long as possible *see details below
2) Keeping your child harnessed as long as possible
In order to do both of these things, it's important to look carefully at the seats before you choose one for your child. If your child is long and lean, the highest weight limit will probably be unimportant for you since your child will outgrow a seat by height. If your child is more solid, you want to be sure the harness will hold the higher weight limit.
There are MANY seats out there which harness up to 40 pounds. Most of these seats will be outgrown before most children hit 40 pounds because the shell of the carseat is short, and kids outgrow them by height. There are also several combination seats out there which say they go to 80 or even 100 pounds, but close inspection reveals that the harness is only rated to 40 pounds; after that the seat must be used as a booster.
I strongly recommend that you purchase a seat for your child with a harnessed weight limit above 40 pounds. These seats have taller shells and higher weight limits, and are much more likely to carry your child to an age and size when s/he can safely ride in a booster seat. This is the current list to the best of my knowledge of all US seats that harness past 40 pounds:
Harness to 80 pounds:
Britax Frontier (converts to a booster up to 100 pounds)
Sunshine Kids Radian80
Harness to 70 pounds:
Harness to 65 pounds:
Graco Nautilus (converts to a booster up to 100 pounds)
Sunshine Kids Radian65
Britax Marathon/Boulevard/Decathlon/Boulevard CS
Safety 1st Apex/Safety 1st Signature/Alpha Elite Apex (requires head support in the vehicle)
Safeguard Child Seat
Harness to 60 pounds:
Safeguard Go (requires vehicle head support and tether)
Harness to 55 pounds:
Fisher Price Safe Voyage Deluxe (discontinued Fall 2007, so can be harder to find)
Harness to 50 pounds:
Evenflo Triumph Advance
The Benefits of Extended Rearfacing
In the majority of crashes, the strongest momentum is going toward the front of the car. When a child is rearfacing, that force pushes the entire body against the carseat shell. The entire head, neck, and spine area is supported by the seat. When a child is forward facing, the same amount of force is distributed over the harnesses instead, and the head lurches forward. In an older child or adult, the spinal column has developed more strength as vertebrae have fused together. Also, the head's size proportionate to the body decreases as the child grows and gets older, so the force is lessened. Except for the driver (who needs to see the road!) we'd all be safer rearfacing.
Children under the age of 2 are definitely safer rearfacing. This article mentions a study that showed children are more than 4 times as likely to be seriously injured when forward facing as rearfacing in some types of crashes, and smaller benefits in other types of crashes. The American Academy of Pediatrics and the National Highway Traffic Safety Administration (NHTSA) both recommend that children should ride rearfacing to the limits of their carseats.
Seats which harness to 35 pounds rearfacing
Sunshine Kids Radian65/Radian80
Britax Marathon/Boulevard/Decathlon/Boulevard CS
Compass True Fit
Evenflo Triumph Advance
Evenflo Titan Advance
Safety 1st Uptown/Avenue
Safety 1st Alpha Omega Elite
Orbit Toddler Seat
More specific information for each seat
Britax Regent: Forward facing only seat which harnesses to 80 pounds. Child must be at least one year old, weigh 22 pounds, and have a seated shoulder height of 12" to ride safely. Pros: Very comfortable seat, very high harness slots, high weight limit. Carries almost any child to a safe booster age, some may be able to move right to the seatbelt when outgrowing the Regent. Cons: Large and sometimes cumbersome seat. Cannot be used on aircraft. Cost: Approx $250
Britax Frontier: Forward facing only seat which harnesses to 80 pounds, then converts to a booster up to 100 pounds. Child must be at least two years old, weigh 25 pounds, be 30" tall, and have a seated shoulder height of 12" to ride safely. Pros: High harness slots, high weight limit, side impact protection, looks like a 'big kid' seat, converts to a booster. Kids like the cupholders. Cons: Many parents have had difficulty getting good installations and good fits as a booster seat. I expect Britax will improve upon this design and make some slight modifications. Cost: Approx $280
Sunshine Kids Radian65/Radian80: Convertible seat which harnesses to 65 or 80 pounds, depending on the model you choose. Rearfacing weight limit 35 pounds. Pros: Very high harness slots, narrow enough to fit 3 across in a small sedan, folds for travel, 8 year expiration (vs normal 6 year), can be tethered rearfacing. Cons: Can be difficult to install in some vehicles, and you must read the manual very closely to know how & when to properly use some unique parts of this seat. Cost: Approx $200 for Radian65, $250 for Radian80
Recaro Como/Signo: Convertible seat which harnesses to 70 pounds. Rearfacing weight limit 35 pounds. Pros: Very high harness slots, high weight limit, narrow seat fits better next to other carseats, can be tethered rearfacing. Cons: Difficult to find in stores to try out seat. Cost: $190 for Como, $200 for Signo.
Graco Nautilus: Forward facing only seat which harnesses to 65 pounds, then converts to a booster to 100 pounds. Pros: Inexpensive, high harness slots, high weight limits, looks like a 'big kid' seat, kids love the cupholder & secret compartment, easy to install, backless booster good for 9 years. Cons: Doesn't come in very many colors (no girly prints). Cost: Approx $150.
Compass/The First Years True Fit: Convertible seat which harnesses to 65 pounds. Rearfacing weight limit 35 pounds. Pros: High harness slots, side impact protection, supportive padding for smaller children, head rest is removable for smaller infants to allow a better fit in vehicles, cover comes off without having to remove harness. Cons: A new seat, so it's less proven, large seat. Cost: Approx $180-$200
Britax Marathon Convertible seats which harness to 65 pounds. Rearfacing weight limit 35 pounds. Pros: High harness slots, high weight limits, side impact protection, extremely easy installation in most vehicles, can be reclined forward facing, comfortable seat for kids, tons of covers to choose from, can be tethered rearfacing. Cons: Expensive. Cost: Approx $250-$280.
Britax Decathlon: Same basic seat as the Marathon with the addition of a supportive infant insert, a button for the harness adjuster, and adjustable crotch strap. Rearfacing weight limit of only 33 pounds. Cost: Approx $295
Britax Boulevard/Boulevard CS: Same basic seat as the Marathon with the addition of True Side Impact Protection via supportive and protective headwings, a supportive infant insert, and a continuous harness adjustment (just turn a knob to adjust the harness to the exact height your child needs without reinstalling the seat). The Boulevard CS is the newest seat and also has a Click and Safe feature where you hear an audible click when the harness is tight enough to safely restrain the child. Cost: Approx $290-$310 for Boulevard, $330 for Boulevard CS.
Fisher Price Safe Voyage: Convertible seat which harnesses to 55 pounds. Rearfacing limit of 33 pounds. This seat is very similar to the Marathon-it was actually made by Britax under the Fisher Price name. Pros: Easy to install, high harness slots, higher weight limits. Cons: Discontinued in fall 2007 so less than 5 years before expiration, only one cover. Currently only available at www.albeebaby.com as far as I know. Cost: $130
Evenflo Triumph Advance: Convertible seat which harnesses to 50 pounds. Rearfacing limit of 35 pounds. Pros: Tall seat, may last thin kids almost as long as the True Fit or Marathon. High rearfacing weight limit. Cons: Some kids may outgrow before they're ready for a booster. Cost: $125-$140
Evenflo Titan Elite: Convertible seat which harnesses to 50 pounds. Rearfacing limit of 35 pounds. Pros: Supportive padding for smaller kids, high rearfacing weight limit. Cons: Low harness slots, probably will be outgrown before kids are ready for a booster. Cost: $90-$100
The following seats have higher RF weight limits, but only harness to 40 lbs.
Safety 1st Avenue/Uptown: Convertible seat which harnesses to 40 lbs. Rearfacing limit of 35 pounds. Pros: Taller shell, high rearfacing weight limit, affordable. Cons: Low forward facing weight limit, need to buy another FF seat before a booster. Cost: $75-$110.
Cosco Scenera: Convertible seat which harnesses to 40 lbs. Rearfacing limit of 35 pounds. Pros: Very affordable to keep kids rearfacing longer. Cons: Very short shell, usually outgrown by age 2-2 1/2, very little padding, can feel flimsy in some vehicles, will need another FF seat before a booster. Cost: $45.