For me, it depends on the weather and what we're doing. I have a couple of pairs of Birkinstock sandals, a pair of Teva sandals and a pair of Nike Straprunners that have all made trips to WDW. I usually take 1-2 pairs of sandals and a pair of sneakers. The Tevas and the Straprunners are great to wear when there's a chance of rain or getting wet - I wore my Tevas all day at DTD in the pouring rain one day in March this year, and was never uncomfortable.
We usually end up going back to the resort sometime during the day, so what I tend to do is wear one pair of sandals in the morning, then when we get back, change to either another pair or my sneakers. I usually wear my sneakers in the evening, but during the day it's just too hot! I also wear my sandals everywhere at home in the summer, so they're well broken in, and I've never had blister problems with them even at WDW.
Hope this helps!
Mur!