What trait makes a better competitive athlete? Humility or Arrogance?

the ideal athlete can be both. humble when not racing, arrogant when they are.

You have set up what is known as a "false dilema" ... sorry, but the world just isn't "either/or".

I've found that humility works best at all times, except when racing, in which case there should be a quiet arrogance spoken only to one's self: You will not beat me today.

Depends on the sport.

Sports like boxing for instance seem to have an abundance of very confident to downright arrogant champions. But it is a sport that really demands total self confidence. There is no time to worry and getting intimidated is the end for you.

So in that case I'd take arrogance.

Some of the teams sports would require more humility to be an effective team member. Football for example. Despite all of Terrell Owens' obvious talent, his inability to shut up has been a huge liability.

For running I'd say somewhere in between. Since it is an individual sport and is therefore pretty selfish, some arrogance won't kill you. But being willing to admit faults would help you improve your game and not race like a moron.

arrogance is being confused with confidance.

example: yesterday at the XC meet, when i got out of the car in the parking lot, i knew i would win. not because i was arrogant about it. not because i knew the competition. i just believed it. i went from the gun and never looked back. i was confidant in my ability, my training and trusted my coach. was i arrogant? no. i wasn't thinking that the other runners shouldn't be in the same race as me. i just thought to myself, "I can win this. the work has been done in practice (mental and physical). i just have to act it out." and i did just that.