I never played outside so I can't really tell if any type is superior when outside. However when not in such conditions, I'd vote for the OLED because that's what the Vita was known for, and that's what games have been designed for at Vita's launch. Gravity Rush is unplayable on a LCD screen in that inferno map.
Anyways, here are pics for you to try and judge by yourself. All at max brightness. Excuse the ghetto setup, that was the only place in my house where the sun directly hit a wall, and I was not going outside to take a picture of my Vitas lol, my neighbours would have wondered what I was doing.
First picture is "not particularly in sunlight". Second picture is "sunlight quite directly on the screens".
As I said, I still never really tried them outside, but from these tests/pictures, I'd say they're equally bad in terms of brightness in bright sunlight, but since the OLED has better contrast it still displays a better picture in such conditions.