We went last month with our 6 month old and 2 year old and took our side-by-side and it worked great. When I was shopping for a double, I looked at both tandems and SBS. I decided I wanted a SBS b/c the tandem reminded me of pushing those giant supermarket race-car carts- it was just too hard. They were also REALLY heavy and bulky when folded. I originally bought a Graco DuoRider SBS. I liked the big baskets and its size when open, but it was too bulky when closed. It was better than the tandems, but still so big that it took up too much trunk space. So, I now have a Maclaren Twin Traveller. It is MUCH smaller than the Graco when folded. This is the stroller we took to WDW.
We stayed at the CR and spent most of our time at the MK. So, we typically were walking or on the monorail. The SBS was great for this. We never had trouble getting on the handicap car of the monorail. When we did take the bus, our stroller was fairly compact and not too much of a "beast" to carry around. I saw a lot of people on the busses with single strollers that were bulkier than our double. However, since we really didn't ride the bus too much, we would have been fine with something that folded bigger. If we rode the bus everywhere, this would have been a bigger issue. On the other hand, if we drove to all the parks, this would not have been an issue at all. So, you may want to consider where you are staying. Lugging a huge, heavy stroller, along with 2 kids and the associated gear onto a bus several times a day can be a nightmare.
I have trouble navigating our double around racks in stores at home that are tightly packed (this is getting worse with the holidays approaching!), but we didn't have any trouble at WDW. I think that Disney does a good job accomodating the needs of wheelchair guests, and in doing so, makes it easy for those of us with SBS strollers.
HTH . . . enjoy your trip.