simple answer is: the mains affect the playability of the ball more than the crosses. IF you're going for spin, then you want strings in the mains that produce more spin.
Thinner strings produce more spin than thicker strings, so you want thinner strings in the mains.
Thicker strings in the crosses only if you want to, but again you'll get better spin if you have thinner strings in the crosses as well. it won't affect it as much as mains, but it will help.