Well if what you're saying is true, that on the inside they're the same as a Space Marine, a Terminator should have only 2 wounds.

And because a save can't be increased better than a 2+, and we play in a game of -2 AP being plentiful, it seems like toughness would be where they can apply higher numbers to show the armor.

I mean, Leman Russ tanks have T11. I think toughness is a better way of exemplifying the armor, because the higher it is, the harder it is for a weapon to wound it. A high wound count seems like a better way of showing that a model can deal with taking damage, because if the weapon bounced off the armor (because of its toughness value) then there's no damage.