They'd both be high-powered, RWD Caddy coupes. Only someone who is dead set on having the retractable hardtop wouldn't cross-shop them...
The XLR-V is roughly twice the price of a CTS-V though, and (once the second-generation XLR-V comes out) will likely have the chassis and luxury to justify being twice the price of a CTS-V.
Theoretically, the A3 would steal sales from the TT, and the 1-Series would steal sales from the Z4, but in reality this doesn't happen. I think the same would be true of the XLR and CTS Coupe.