It is standard policy at most dealers that they don't loan out high end cars for several obvious reasons...
1) Most drivers are not capable o handling M3,M5, ZO6, ZR1, CTS-V,...etc., at speed.
2) These cars are too expensive to loan out..
3) Perspective buyers do not want an M3, ZO6, etc, with "loaner" miles on it, would you ?
4) Insurance on a "loaner" car is questionable....Most dealer insurance companies now, refuse to cover "loaner" cars, that is why 99% of them use Enterprise, or some other car rental company......
5) I know of one situation where a "good friend" of the dealers son, who was the GM of the dealership, was given a ZL1 Camaro when his ZR1 Corvette was in for service. This guy knew what he was doing and he let the ZL1 get away from him. He ended up writing out a check for about $15,000 for new front end, new wheels and tires, and incidentals. They had a hell of a time selling that car, even with a big discount, and most ZL1's come with a big mark-up.....................
On looks alone, the ATS is the winner for me. As a current DTS owner and Cadillac owner since 1970, I do not care for the XTS , especially the styling. If only the ATS were a bigger car.