what do you think(for flow)?

[Aquattro]

I think the MAK4 will give you more flow by the time it gets to the tank. It handles pressure a bit better and is therefore a more versatile pump. Same price range as the MAG as well.
As for the sea swirl, by the time the MAG 12 gets water to come out the sea swirl, your 1200gph will hqave dropped a fair bit. Even on a closed loop, you have head loss due to friction in the pipes. Outside of a closed loop, head loss is even greater.