Our children are 3 1/2 and 6 1/2. We just had this discussion last evening. We were really hoping to do this trip with just the umbrella stroller and have them take turns if the 6 1/2 gets tired from time to time. She barely fits into it, but it would still give her a break LOL!
Well, our daugther, the 6 1/2 has never been much of a walker. She complains and whines and it isn't much fun for anybody. THe times you need the stroller the most are at the end of the night when they are exhausted to get from your last ride/attraction to the park entrance, and then from their to your transportation and then from their to your room (sometimes they are practically passing out by then!). No way, we could carry her then, or deal with the whining!
Also, another poster had a great point. It's one thing at MK where we practically just run from attraction to attraction in Fantasyland. There, the stroller might sit half the day. But Epcot is a LOT of walking. Even Hollywood studios.....she might be interesting in the Ariel show, but then it's quite a long walk to the next thing she might be interested in.
So, it looks like yet another year of hubby schlepping that big stroller on and off the bus. But I think it's worth it to keep the kids happy

We sold our double, she barely fit in the back of it anymore! So we'll be using our sit and stand. No, it doesn't offer a lot of room up front for the 3 year old, and taking a nap would not be too comfortable, but they can both get off their feet! And there sure is something to having a place for all your stuff. With our youngest, we still need to cart around extra outfit, snacks, wipes. etc. WIth the umbrella stroller......you all know, it would just tip over constantly with bags hanging on the back!
So, I think we've made our decision..........it really is a lot of walking! Aren't the adults feet tired at the end of the days? Imagine their little feet and legs!