That's a really useful comparison; it's not often you hear exactly the same input though two different speakers.
I know that the G12-75 comes in for a lot of stick sometimes but it has it's place in the general scheme of thing otherwise Celestion wouldn't continue making it.
If, for instance, you play a Les Paul from the really shitety period when Gibson, and Epiphone; by association, were overly obsessed with muddy middle and bass tones the extended top of the G12-75 could be the answer to a maidens prayer