K.M.
We have the sit-n-stand and are very happy with it! We got it when we had a newborn and almost 3 year old. Now the kids are 2 and almost 5 and it still works great. My 5 year old can ride if he wants or hop off:) Baby #3 is due in July and it will be perfect for baby and 2.5 year old.
We looked at a bunch of strollers but most double strollers were too heavy. I'm petite and have to lift it into a SUV. The sit-n-stand was one of the lightest ones. I'm now 7 months prego and can still lift it into the back of my SUV.
My sister has a Graco double stroller (two full seats, etc) and hates it. She calls it the18-wheeler and only uses it when she has to. She said it's hard to lift the older child into the back once they are 3 or 4 years old...she wishes they could just hop on by themselves.
A few friends said the side-by-side was great for neighborhood walks, but horrible in stores...too wide to fit through doors and aisles. But, I never tried one.