I voted loot, because that used to be my drive: getting gold keys and a chance for a helm.
But the recent introduction of the Cyrodill vendor has drastically reduced the amount of time I spend in dungeons.
Gold keys have no value to me anymore, and I don't have the will to farm a specific dungeon for a helm anymore either.
I bought all the Helmets I need.
The only dungeons I still run on a semi-regular basis are vWGT and vICP.
And those runs are loot driven (non Monster sets), although I like the challenge.