The 6 piece ones are definitely better, I used to use the 2 piece bearings, they aren't too bad if you actually measure the thrust and make sure everything is in spec, I always used to wack both ends of the crank with a rubber mallet to move the crank and than remeasure the thrust. And in that vortex thread, IMO its not the surface area at all that has anything to do with the bearings being better, its just the fact that the thrust parts of the bearing aren't attached so they can move without damaging the main journal/cap.