1st question, not sure if I know of an exercise that would target the center more then the upper and lower. But then again I'm pretty sure you don't need to target that. Just make sure your working your core effectively. Personally I like using the ab wheel, db side bends, and back raises.
2nd, Utlize both, but since you didn't list numbers, I'll say don't go too extreme in either direction. For example, you don't want to be doing heavy triples, or sets of 50 either. I like to stay in the 8-12 and 15-20 range.