That is if you lease the BMW. My dad likes to own things outright.
I disagree with you on the ATS. The interior is better than the 3 series. BMW's have always been about function over form. Not a bad thing, just not visually appealing. Materials are not bad either. The ATS interior design is better and the materials are good too. I love the morello red interior with the carbon fiber trim. It's a nice understated red where BMW's red interior is bright and flashy.
When I first drove the ATS back in October when the ATS was just hitting dealer lots, yeah CUE was laggy and unresponsive. But, I drove it again last month and Cadillac has updated CUE with massive improvements. It's far more responsive and smooth now. The only thing I find to be lagging still is the zoom in/out on the map in navigation.
The biggest thing I have observed from videos complaining about CUE and from my own dad trying to use it is with the HVAC controls being completely touch controls. People are pressing the metal accents thinking it's part of the button or something. So when it doesn't immediately respond, it frustrates them. They need to know/remember the metal accents are just for decoration. They are not apart of the controls. Want to increase temperature? Press the red arrow only and the haptic feedback will tell you it registered.
As for driving, the only thing I find the ATS to be lacking compared to my dads 335xi is the engine. The 3.6 V6 is not a bad motor. But, the N54 is just in a different league due to the twin turbo's and all the torque being made at 1400 RPM. It just gives you a great experience. But, I found the chassis on the ATS especially with the magnetic shocks( only available on RWD models

) to be better than the 335xi. The steering felt more precise and when in the corners, my smile was bigger in the ATS than the 335xi.
As for straight line times which is good timing, the ATS's slower times looks to be all GM's software related. Motortrend recently compared a tuned 335i and a tuned ATS 3.6. The BMW had a ECM tune which increased power, etc. The ATS only had its shift points redone. The result? The ATS was only .1 second behind the 335i. Both had modified suspensions too. Maybe that played a bit of a role, but looks like a simple software change on the ATS will improve its straight line performance.
YouTube:
video
As for the Lexus IS... It's ugly. And it's a Toyota. Toyota's are just soulless cars with a few exceptions.
Also, I have a problem with the way the magazines reviewed these cars. It's entirely inconsistent. When the ATS came out and was compared to the F30 3 series, everyone loved it and preferred the ATS over the 3 series. Then later comparos came out and the ATS was either last or close to last. How can MT, C&D, etc say the ATS is better then have it lose so easily later? It pisses me off.