Kos didn't improve that much. That's only what the people who'd been rubbishing him for a couple of high profile errors started saying when they finally worked out how good he was. He was turning out good performances from when he arrived and was generally at a better level than Gabriel has yet shown.
I would go for Per over Gabriel. Gabriel's age isn't terribly relevant, as he's only "one for the future" if he can step up a fair bit which has yet to be seen.
I'm never too sure if Gabriel is genuinely dumb, unfortunately for him he's a bit unprepossessing and looks thick, and he does a lot of erratic defending. If it's just a psychological thing then perhaps he can find another gear.
