I got this from another site. By the way, I used the "P" bearings in my 331 build.

'H' bearings are sometimes considered race bearings. They are made of harder material to take the heavy pounding that high rpm/racing cranks have to live through. They are also narrower or heavily chamfered so they do not contact the larger shoulder or radius on the journals of alot of aftermarket forged cranks. Since they are harder, they do not like dirt particles in the oil. The 'P' bearings are still a high performance bearing, but their outer layer of material (the side against the journal) is softer to 'absorb' dirt particles in your oil. 'P' bearings are probably your best bet, unless you have a race style crank that needs a chamfered type bearing. If you do use the 'H' bearings, you will want use a high quality filter (with no bypass) and change your oil more often. You may also want to place a couple magnets near the oil pump pickup to catch metal particles in your oil.