I'm a total stroller addict, so I've been through a few doubles. I'll give you my two cents! My kids are 13 mos apart, so my son was 13 mos when my daughter was born (just by way of refernce)
I started with the Combi Twin (side by side) b/c it folded up really small and was really lightweight. It's not made to take a carseat, but I used my Britax Companion in it, and unless you plan on going off-road with it, it's not going to fall out. However, I found that it was flimsy after awhile, and cramped for my toddler. Once the baby moved out of the carseat and they were next to each other, the baby drove my toddler crazy b/c she grabbed at him all the time. I just felt there wasn't enough separation. There were also no cupholders and storage was paltry at best. NOT my favorite stroller, but it worked in a pinch.
Next I got the Bob Revoluion Duallie. LOVE, but it's really big and heavy, so not the best for transporting in your car, etc. Good for taking walks in your neighborhood.
I have heard wonderful things about the Graco Duoglider. Everyone I know who has one LOVES it. It folds up fairly small (for a double) and isn't ridiculously heavy. It's front/back, so the kids can't get into each other's hair when they get older. It does have two traditional seats in front and back, and will take just about any carseat.
I tried a more expensive stroller (the Jane Powertwin) at one point, but while I found it wonderful to maneuver, it was a pain to fold and stow, and quite heavy. Anything where you have to remove the seats to fold is a pain if you stow it a lot in your car.
I ended up with the Sit & Stand from BabyTrend now that my toddler is older. I liked it better than the Joovy b/c the seat is actually bigger, so he can sit when there is a carseat in front. With the joovy, if you have a carrier in front, the backseat is basically non-existent. Plus the Baby Trend has better storage than the Joovy. Finally, it's about $100 cheaper than the Joovy. No brainer to me. I LOVE the S&S now, and my toddler loves it too, since he doesn't like riding in a traditional "stroller" anymore. It folds up surprisingly small for the size it is when it's open, and while I wouldn't call it light, it's not as heavy as the Bob. If my son had been old enough when the baby was born, I would have gotten the S&S from the get-go.
Hope this is helpful!