I will tell you what NOT to get!
I have the Graco Quattro Tour Double. It drove like a dream when I tested it out in the store - throw 2 kids and diaper bags in there and it weighs a TON!!! It's also freaking HUGE, it's like driving around a school bus.
Since your son is already 3, you might consider a Sit N' Stand. I wish I could afford to get a new double, but even if I could, a sit n' stand is not good for me since my next youngest will only be 18 months.
I've been trying to find a sit n stand second hand (I beat them crap out of my strollers so I just can't justify brand new), but I saw one out and about recently and it has a huge wheelbase. I had a Jeep double (front to back) and resold it because it was so cumbersome to maneuver and this looked just as big. I happened to see the Joovy Caboose (same idea as sit n stand) at the same place and I bet it was 5" shorter and the mom was pushing it with one hand. So now I'm leaning towards one of those if I can find one (and if it can hold my car seat).
I have a Schwinn Turismo jogger too which I love but never take it anywhere except running because it's so huge that it won't fit in my trunk and my 3.5 year old generally walks most of the time anyway.
Yeah, I have a hard time justifying spending $600 on a stroller! But it depends how much you use it I guess. I know some people walk nearly everywhere, but I only use my stroller for: in the summer, we walk to the park, and in the winter, I will use it in the mall.
Maybe I shouldn't respond since I've only used a stroller like 2-3 times in the past 4 years! Anyway, I like strollers that are lightweight and fold up small. The only stroller I have these days is an umbrella stroller that I use on rare occasions for a toddler. I always wear my youngest child - whoever that may be at the time. That's a personal preference I suppose though. But I do find baby wearing MUCH easier then strolling around any kind of stroller. One thing you might want to consider is a wagon. We use that for trips to the zoo and such. Any combination of stuff and kids can easily be accommodated. I find it easier to pull then to push, and the wagon just seems much easier to maneuver then a stroller for the most part. If I was ever going to get a double stroller I would likely go with something like this...
Strollers | Wayfair
But again, feel free to take my opinion with a grain of salt since I'm really not a stroller person.