... I'd vote for February! I've been in late January, early Feb. and mid Feb. and although there were some coldish days in that timeframe (mid 60s during the day), most of the days were in the 70s-80s and sunny. The crowds on all trips (except over President's Day weekend) were minimal. I've also been in mid-May (18-22) and have several friends who went the first week of May. We all found the crowds to be moderate (definitely not as low as in February) and the heat to be rather unpleasant for touring the parks. I would probably avoid going in May in the future for these reasons, though the flower and garden festival is pretty. If you don't mind the heat (mid 90s) and moderate crowds, however, May may be the better choice. Also, if you don't want to run the risk of encountering a few days in the 60s or 70s, then May may be a better choice. I have never not been able to swim at all during a February trip, though there were a few days on 1 trip when it was too cold for me and dh (though not for our kids!!).
Whatever you decide, I'm sure you'll have a GREAT time!!
Good luck deciding,
Sarah