my oldest is 3.5 and hes looks almost too big for the double stroller now (we have a graco duoglider). the leg rests are really short so his knees are up high when he sits. i really only use it when i have to. the sit n stand would prob be much more appropriate for his age.
I am giong to go and buy a babytrend sit n stand today actually. I found one that converts from a double stroller to a sit n stand. So that will be the one I am going with. It gets decent ratings and is cheaper. But I have noticed most double strollers only get decent ratings due to there size and weight!
i really like my sit n stand. i have 3 kids 4 2 and 1 and i put all of them on it, plus when the new baby comes i can attach the carseat to the front. currently my youngest sits in the front, my second sits on the back seat thing facing forward with her feet in the basket and my oldest stands on the back. it folds up just about as small as a single stroller and is pretty easy to manuver