If you are going to make it simply a MH race, it will attract only GPUmax buyers, and those wont stick around. If you are going to make a competition of sorts, Id make sure everyone has at least a chance of winning, so you can attract any kind of miner and they might stick around.

Id make a mining raffle, where every submitted share is a ticket giving a chance to win. YOu might even allow people to buy additional tickets, although I assume the price wont make it worth it.

As for the prizes; well, the higher the better . Adjust the chance of winning to make it worth your while. Maybe you can throw in some feebies, like allowing the winner to chose some text to embed in the block header for posterity?