I agonized over the same thing. Our son is almost 2 and daughter is 3 months. I wanted the sit and stand, but knew it wouldn't hold up with the amount of walking we do. Plus, I was worried that he wouldn't stay in it because he is a busy boy.
What drove me to purchase was trying to get in and out of places with both of them. I would end up carrying a carseat, a toddler and a diaper bag if we were in a parking lot or pushing and overloaded bulky stroller with the carseat in it, holding his hand, and then the baby would get fussy so I would be pushing an empty stroller, have the baby in the Moby wrap, and holding my toddlers hand. I felt like it wasn't safe sometimes!
I finally landed on the Bumbleride Indie Twin. It comes with a carseat adapter and I also bought two snack packs which hold sippy cups and snacks when they are in the stroller. This was a huge purchase for us, I cannot believe how expensive these things are! BUT, after three months of trying to juggle both kids when we were walking in and out of places like the mall or the children's museum, I just really felt like we needed it.
I am so in love with the stroller. It is really easy to push and it fits through doors and aisles easily. It is really versatile.
I have to laugh because the only complaints that I have heard are that it is big and heavy. Well, it is. But it's a double stroller. And with two kids under two I'm not exactly light on my feet these days anyway! LOL. I feel like I pack up the house when we go out. =) So to me that is not a problem.
Anyway, that's what I landed on and believe me I would not have forked out that kind of money unless I really felt we needed it.
Good luck on your decision! Having two is so wonderful, you will really enjoy life with two!