I think I am in the minortiy here. If your main reason for bringing them is so they don't feel left out, I say leave them home. There are lots of kids who do not dress up.
I completely understand your situation. I went back and forth with the same question for our trip in Dec. and ultimately decided not to bring them. My DDs (2.5, 5, 6) play dress up all the time at home. But at WDW they wore Disney shirts every day (simple ones, not custom) and that was enough for them. I was a little nervous at the Storybook Princess dinner because there were a lot of girls dressed up, but they never once got upset or even asked about it. They were just incredibly excited to meet the princesses.
For me, the benefits (cute pictures, won't feel left out) did not outweigh the negatives (not as comfortable for my girls as regular clothes, the logistics and effort or transporting them to both FL and WDW, storing them during the day at the park, etc. etc.)
You know your kids better than anyone, and if you think they would enjoy it, then go for it. Personally, I am glad I did not bring them.