Hmm,
For Nov and Dec separately. For each thousand units the Xbox One outsells the PS4 in the US, I'll give you 5 credits. For each thousand units the PS4 outsells the Xbox One in the US, you'll give me 5 credits. An additional 50 credits per month just to winner.
Sound fair?
How about 100 for each month. I don't have a large bank account. And if xbox doesn't pick up it could be wiped out in just one month.