You can look at crowd calendars on-line. I'm guessing the crowds would be about the same. I would definitely choose March, even if the crowds are a bit more. You would be much more likely to have just lovely weather(trumps dealing with crowds IMHO). In May, it starts getting pretty hot and much more likely to have the occasional thundershower in May. March would be likely comfortable temps, not so many storms and yet still hot enough to enjoy the pool.