From my experience, I would choose the Sit-n-Stand over any other. When DS#2 was born, DS#1 was 16 months old so I figured a double stroller was a MUST.
I originally got a side by side double stroller....here is a pic.
Anyway, I really liked it, but it was not until I traded it for the Sit-n-Stand about a year and a half later that I realized I really hated it in comparison. A side by side is PERFECT if you have twins or two children about the same size. It was VERY hard to manuiver when the wieght on both sides is not that even. What was so great about the Sit-n-Stand is that fact that the older child can sit, stand or get in and out of the stroller with ease and if your 2 yr old is anything like mine, every time you stop to check out an animal at the zoo, they want out to see too kwim? Then, as far as the baby is concerned, they are commfy in the front and most come with the bar attachement so you can snap an infant carrier into it.
I will say, my girlfriend just had her second child and her oldest is almost 3. He is a big boy weighing almost 50lbs and she just spent $500+ on the Bob double jogging stroller and the weight distribution is NOT a problem with this stroller, but for as much as I used my double stroller, I would NEVER pay what she did