Hmm, I don't have time to look up the precise wording the rulebook gives on this, but I know for sure that when we have multiple saves, GW has given us permission(through the FAQ, I believe), to decide for ourselves what our "best" save means.
So for instance, if you had a 6+ invuln and a 5+ armour save, and you were being shot with AP5 weapons, you might still be able to choose the 5+ armour save as, in your estimation, being your "best" armour save for the given situation.
Again, I can't confirm this at the moment due to time restraints, but it's at least a hint on where to begin looking if you'd like to research the wording.