Ideally... Koshe has already mentioned that there was an idea to completely abandon independently upgradeable stats and tie everything to skills and abilities. In theory, such a system could be made very flexible, but it requires a full skill tree and a huge amount of time to think it all through.
What does this mean?
"Average" (10 stamina) has 100 HP.
"Warrior" (15 stamina) has 150 HP.
"Tank" (25 stamina) has 250 HP.
A warrior is one and a half times tougher than an average unit. A tank is two-thirds tougher than a warrior and two and a half times tougher than an average unit.
First draft:
"Average" (10 stamina) has the same 100 HP.
"Warrior" (15 stamina) has 225 HP.
"Tank" (20 stamina) has 475 HP.
That is, a warrior is more than twice as tough as an average unit. A tank is twice as tough as a warrior and almost five times tougher than an average unit. The stamina cap, however, is lower.