It is the age old question, and truly a matter of personal choice, experience and opinion. If you are fairly new I would suggest a gelding as you don't have to worry about any type of "breeding imperative" -- but it depends on what suits YOU.

I always preferred mares over the boys when I was younger, but they can be moody -- but then so can I, so it seemed to make sense to me! LOL

What kind of competition are you talking about?