Attack++ (ie Assault (L)) gives 6 + (skill_lvl / 2) assault.

Attack+ (ie Assault (M)) gives 3 + (skill_lvl / 2) assault, while HP+ (ie Defender (M)) gives 3 + (skill_lvl / 2) HP.

So the latter would really only give up 3% assault in return for the HP. That's normally a worthwhile trade.