Not comparing it with the Viper, two completely different vehicles, with different "purposes".
You have a Caddy, I just figured the luxury ride coupled with the performance would be attractive. And the hard top convertible on the XLR is way cool!!
If I already had a Viper, and a Viper truck, the Caddy would be a nice choice because the Viper, the Truck and the Vette don't compare to the comfort of the XLR. IMHO I would probably go with the XLR too, not the V. 302 HP in that car performs pretty well. It's not a race car, and was not intended to be. At least not like the Viper and the Vette. It's super classy, and gives an amazingly quiet, comfortable and still sporty ride.
It's your money, I hope you make the choice that is right for you. Because you are the only one that has to be happy with it!