I have no personal experience with either. But here is some great reading material if you're in the market for a scope. And he's very positive about the NXS. He even relates story of a friend who prefers the NXS over the ATACR. This guy goes to great lengths to be neutral on presenting the data.
If you absolutely need the added elevation adjustment the ATACR offers then go for it. Keep in mind that it's a 34mm tube so rings cost more than the 30mm rings the NXS will need.
Another notable opyion in that price range you might want to look into is the Bushnell Elite Tac stuff.