return above or under

[Basile]

I'm in the process of doing putting my plumbing together and for my return i'm undecide, i've seen people do either their return above the water churning the water line and some plunging it under the waterline. My tank is rimless and without canopy so that in mind i was thinking a screen on top that i take off when i have guest. If so it would mean above water no? How's your set up and why? Plus is having your return churning the surface of the water a plus for gas exchange?