From what I've read and heard the Merc 525 is a better engine but should also cost more than the GM HP3. I believe the Merc motor has better internal components, pistons & rods.
BTW, upgrading to the Merc 525EFI from the HP500, wholesale cost difference is $4,000/engine.
Merc 525EFI Picture
Merc 525EFI Info