I might be really very biased, but I would certainly advise holding a contest over at
yaxay if purely because we have many more designers over there and a longer history of successful contests than here.
From my view (which may be wrong), the contests over here suffer from the lack of numbers in the Design Team. Sure there might be quite a few people in here now, but remember there is only a percentage who will take on something like a vB skin (eg, I won't). Also, those vB designers over here also frequent Yaxay.
I have less experience with custom vB design, but $200 is certainly not a generous offer which would attract those more experienced and talented custom vB designers but is a healthy amount to offer in a contest. You also explicitly stated in your first post that you'd like to get the most to choose from. Alot of custom designers won't necessarily put a number of finished/semi-finished ideas together just to pitch to a client.