The bullet weight is up to you. But for comparing the BC's of the bullets, generally your heavy for caliber bullets will yield the highest BC for longer range shooting. If you do not plan to shoot 400+ yards, then BC really doesn't matter that much. Go with what shoots the best. The problem in the WSSM case is, the round needs to be really short, and a heavy 25 caliber bullet would be hard to fit in the magazine.
If you run the numbers on a light weight fast bullet and compare the drop and wind drift to a heavier bullet starting out slower, the heavier bullet will begin to have less drop and much less wind drift than the lighter bullet does, b'c the heavier bullet is more stream-lined, and does not bleed off speed as fast as the lighter bullet does.