Well, I'm no expert by any means but I did just look into the same question (very extensively myself). After MUCH research, I decided on the Graco Quattro Duo. It actually just arrived on my doorstep last evening, and I haven't even opened the box yet.
After my search, I decided on either the Graco Quattro Duo or the One Step Ahead double elite/sit and stand. I loved that the One Step Ahead could be either a double stroller or you could remove the back seat and it could become a sit and stand. I thought that would be the answer to all of our problems, but in the end, I changed my mind. Here's what I came up with:
Pros for the One Step Ahead: growing value, LOVED that we could use it as either double or sit and stand, looked easy to fold, was lighter in weight (not by much), fit the Graco carseat, got good reviews, and love the One Step Ahead brand. Cons: the storage basket (although large) is quite inacessable once there are kids in the seats, the toddler goes in the back seat (so can't see as well), the toddler does not have a snack tray, the toddler's feet dangle into the storage basket (no leg room at all), and the kids face each other (my son would bug the baby...this could be a "pro" for others.)
For the Quattro Duo: Pros: Like my single stoller/travel system that is a Graco, know it will fit the carseat, available in the same fabric as the carseat (not a big deal but nice), looks easier to steer, older child sits in front seat and has a snack/cup holder, back seat is elevated so when we don't use car seat any more both kids can see out, large storage basket that folds down for easier access, small storage basket under front seat to store more supplies, folds compactly, and looks easier to get kids in/out. Cons: doesn't have the sit and stand feature, seems heavier.
I wouldn't be terribly surprised if we end up with both a sit and stand and a double stroller in the future, but for now the Quattro seemed to fit our needs better. However, it all depends on what you are looking for and how often/for what you are going to use it. Good luck!