P series are designed for stock style cranks. H series are narrower and chamfered on the edges for racing cranks. Are also supposed to be a little harder metal..

 

H bearings, although they can be used on stock crank combos, can be problematic when bearing clearances are setup on the loose side. We typically stick to the P bearings for a stock crank setup. Clevite are nice, and ACL and King get used here and there when the Clevite bearings won't give us the proper clearance (one standard bearing to the next can vary quite a bit one brand to the next, and one bearing to the next)!