We went last Sept when our two were 3.5 (45inches then) and 7 months. I debated between a single and our sit-n-stand then. Our 3.5 barely used a stroller, only somewhat at Sea World and a long day at the zoo. I ended up bring our single and a front pack (knowing WDW is way longer and more tiring than both). DS#1 was in the stroller way more than I thought he would be, especially as the week wore on.
This May the boys will be 4 & 14m and I'm expecting #3; I'm still not bringing the sit-n-stand as it is bulky when folded and harder to push. I'm bringing our double jogger. It is the easiest stroller I have ever pushed, way better than umbrella's (which at 5'8", I find too short even with extenders). It also has a big basket and storage bags, which will be nice for convience. And each seat reclines completely, individually. Plus other extras that I don't really use at home on walks, but will come in very handy at WDW. Another bonus, it's a stroller they know and will be comfortable in.
You can always bring your new stroller and if it isn't working out, rent a double or then buy a cheap umbrella.
