I drove the ATS when they first hit dealer showrooms. It was a 3.6 AWD model with the auto transmission. I wanted to test the 2.0T RWD with the manual but they weren't out yet.
I have a 2006 e90 with sport package and 6MT right now. I have also driven the F30 328i with the auto.
To be honest both made me want to keep my (or get a newer) e90. The F30 felt too big, the ATS handled better than the F30. It felt more tucked and planted and also more tossable. The F30 wasn't bad, but it felt more like a G37. It pushed back if you really threw it into a turn, but the e90 and ATS are more "eager".
The interiors on both were kind of so so in different ways. My impression of the F30 interior is that it doesn't flow quite right. I don't love the color combos and the "lines" thing BMW is doing. The GM interior flows well and feels more like it "wraps" you, but I'm not a big fan of their use of shiny piano black trim.
The GM v6 had lots of power, but it was (expectedly) not as refined as the N52/4/5. The ride on both the F30 and the ATS was better than my e90, but both use RFT which annoys me. GM seems to be using Michelin RFTs though, so good on them.
The F30 livens up a bit in "sport" mode, but you still get the feeling that its just a bigger car. I think I'll test the ATS again when they have the 2.0 RWD available. It was really quite an impressive car.