I liked going during Thanksgiving becausse the weather was cold and I don't really like hot weather.I don't know what March is like, but I doubt it is that hot. You never know with Florida though.
We were at US and IOA on Thanksgiving Day and thought that only a few people were at US, but quite a few were at IOA. I can't remember exactly how long the rides were, but I'll tell you what I remember... We got there first thing and covered almost all the major rides at least once at US before 11am. Nothing was more than a 15 minute wait, but we only stayed there until 2pm or so. I don't like a lot of shows so we moved on to IOA. Since it was my first visit, we did quite a bit of wandering and rode some of the smaller rides with about 20 minute wait like Cat in the Hat. If you would have been waitng for the Hulk the wait would have been at least an hour, Spidey was running over an hour also, but the singles line moved pretty fast. Compared to Disney, Universal was a million times better from what we heard about lines. We ended up at Animal Kingdom for the day after Thanksgiving and loved it. I don't know how busy it normally is, but compared to US it seemed like huge crowds! We used Fastpass, can't say enough about it, and it worked out fine. We saw ALL and I mean ALL the attractions by 4:30pm. (not including shows) We did spend quite a bit of time with the animal encoounters. Then, on Saturday after Thanksgiving the crowds seemed similar to Thanksgiging at IOA, but instead we got there just as they opened and used the early entry. Some of the rides like Dudley were no wait at all and Jurassic was 10 minutes. Early on in the day at that season no one wanted to go on those rides. We covered all the rides easily that we missed before and then did repeats of our favorites. I think we went on Spidey 3 times.
The big plus for us was vacation days. My hubby only has so many days so we had to go over a holiday. I think if you were going for that reason this time of year is great. The only other days with that many off are July 4th and Christmas both are super busy! Also, I liked seeing the Christmas decorations. If days were not an issue and you liked warmer weather I would try spring time, if you can find a time not too crowded and enjoy it! No matter when you go, it's great!