We just got home yesterday. We were in the parks March 10-16th, and it was hot. It was 86-90 every day and not a drop of rain, which was forecast. Crowds grew as the week went, and by Monday the 16th, at Magic Kingdom it seemed much more crowded than it had the previous week. We were still able to get a lot done the first 2 hours at Magic Kingdom, but by afternoon it seemed more crowded than summer! We were at Universal on Sunday, when an employee told us it was so busy because this week (of the 16th) was the local Florida spring break.
For crowds, I would choose September! Weather I believe will always be hot in September, which I'm ok with personally, and would be easier to pack for. This March trip was so hard to pack, when the weather forecast called for anywhere from 73-88 highs with chance of rain each day and supposed to be cold at night. The forecast ended up being completely wrong, but no way to know that ahead of time, so I packed for everything and it was a pain!