It's an easy choice, assuming that 205/50/15 size.... the 15x8 will be faster. The few pounds per corner would make worse ride quality in the real world but for a race car is not enough weight to make much performance difference here. In contrast, for a few extra pounds the 15x8 has BIG advantage with significantly more stable contact patch, and more tire actually on the ground, etc. The better turn-in response will be more confidence inspiring, car will be easier to drive fast, etc.
Tire not stretched at all, sidewalls will be nice and square, that's an eight inch wide tire on an eight inch wide rim....which means tire will be happy and wear more evenly and perform more consistently throughout tire life, etc.