I feel that the .54 is a lot more versatile and can do a lot more under a lot more conditions, but for some reason the .50 seems to be a lot more popular. This may be due to the more easily available accessories as well as the general perception that I have seen among non-ML shooters that the .50 caliber is a big bore.
The Deerstalker has a pretty fast twist though because it is designed to shoot sabots and conicals, especially sabots. Keep that in mind if their are any restrictions on ammo in your state.
If the gun were for me, I'd want the .54, but not the 1:32 wist rifling, but since it could be won by anybody you may want to look into the idea of making it available in the winners choice of caliber.
I would suggest that you get one in each caliber, and give the winner his choice, then send me the one that is left. I won't gripe about the caliber of a free gun.
Actually a kind of crazy idea just came to me: (I'm a little punchy at 3:00 am so bear with me here). If you can make the necassary arrangements with a local shop, give the winner the choice of GPR or GPH in .50 or .54 caliber, and either left or right hand models. Now this should satisfy anybody who may possibly win.