I'm not sure we're distinguishing properly between "Hit Points" and "Health Points". In the first case, HP is an abstraction designed to take into account things like dodging, stamina and luck. This number can be incremented as skills and conditioning improve. A guy can swing an axe at you five times, and your "hit points" are sufficient to avoid being hit. So you're still at 100% combat effective status: You run, jump and fight as usual.

If we're dealing with "Health Points", then the number represents the physical harm being done to your body. In this case, a percentage is more appropriate. When that guy actually manages to connect with his axe, it's put a big hole in your body, damaging muscle tissue, possible severing limbs, and causing shock and blood loss. This will have an immediate effect on you. You'll slow down, or lose your ability to shoot a rifle well, or whatever.

Make sure we're all talking about the same thing. If dodging, stamina and luck are simulated in other ways, then go with health points, and have a large portion of attacks be unsuccessful. If you're going to be abstracting those other attributes, then go with hit points, and have them gradually decrement until your character gets killed.

Whether you increase HP by actually changing the number or by providing 'resistance' that reduces how much damage your HP take when your SP are used up doesn't really matter since they have mostly the same result. I was just suggesting that there should be a way to increase your 'effective HP' over time at a much lower rate than you increase your SP.

Really though, I like the idea of a more abstract wound system for HP and regular numbers for SP. It would need to have wound combining, and the size of the wound could depend on how much overflow damage you receive, but it would be simple to design. Also, maybe critical hits automatically do most of their damage directly to HP and that is what makes them so great (instead of the common x2 damage or whatever).

How about testing this out to decide? Make a pen & paper model really quickly and play through a few scenarios. See if increasing either stat works for what you're going for.