I've worn Merrell sandals before, as well as the classic Birkenstock sandals. Both were super comfy. I've never worn Crocs to Disney World, but I have worn them before while vacationing in Costa Rica (they were the Nile style), and I loved them as well.
My best advice though, is to start wearing your sandals at least a week before you leave, around the house, or outside, just to make sure they are comfortable for YOUR feet, and to break them in.
Have fun!
EDIT: Ooops! I just remembered, when I had my birkenstocks, I got blisters where the straps where, in fact I have a scar on one foot from them. Once I broke them in, they were comfortable though.