I made a thread on this in the past but am now between these two cars. I don't need the performance advantage of the IS350 so to keep pricing close its the 250 vs the 2.0T ATS.
-Older looking cluster
-Nice interior but a lot of gloss
-Cadillac App and factory remote start
-Rain sense wipers
-Decent power out of 2.0T
-Solid fuel economy
-F-sport eliminates a few nice features such as memory seats, ventilated seats, and rain sense wipers
-Pretty aggressive styling and an awesome premium feeling interior
-No factory remote start
-NuLuxe instead of leather
-Different driving modes
-Awesome LFA style cluster
-Great ride/cabin feel
Unfortunately, Lexus removed some nice features when you select the F-Sport package, but I feel like the car is much to 'ho-hum' without it.
MSRP for these two are going to be pretty similar with the Lexus coming in a few K cheaper, however when the programs for the Cadillac come out next week I am expecting to pay ~$50 less per month for the ATS than the IS, all things being equal.