I have 3 kids 5, 3 and 8 weeks. When my 2nd was born I bought a graco double stroller and loved it. Now with the oldest still wanting to sit at some point, I bought a sit and stand and it works very well. I have traveled with it (San Diego and LA from FL) and just quickly take the back seat out to make the stand part. I put the infant seat in the back (there is an attachment for the front but I just put the seat flat and the carrier there) and if either girl wants to sit in front it is a little tougher to turn but still can be pushed. Of course at 45 and 27 pounds if either girls sits in most strollers it will be harder to turn.
The sit and stand folds pretty small for a double, rolls easy when folded and actually has quite a bit of room for storage. We have taken it everywhere, Disney, plane, car...and it still works well. :-)
I would go to a store and try them out though and bring your infant carrier so you can test it out with you kids, fold it up, roll it...my husband had a ball testing stuff before we bought it! :) Good luck!