German Performance cars should only be purchased if you're absolutely lovestruck. Thats my take at least.
Personally I'd go with the CTS-V, especially since you're going to be driving it heavily and during poor weather conditions. If you've never owned a Cadillac before they are cushy, plush and extremely comfortable. Also they respond well to a wide range of conditions. While slightly more powerful, the same cannot be said for the Camaro. Last but not least, that V-Grille is just gangster.
If you do end up getting the V do yourself a huge favor and order a FSM (Factory Service Manual) set directly from GM. They are worth their weight in gold :thumbup:
As for the Diff problems, I remember them having all kinds of issues when they first came out ('04-06). I maintained a constant presence on the Cadillac boards (Caddyinfo / CadillacOwners) around that time frame and can recall all kinds of people bitching about Diff's going out within 40k miles. As I recall GM gave people absolutely no-end-of-shit about replacing them under warranty. For some reason I thought they fixed the Diff Issue around '07? EDIT:
From what I remember the 04/05 rear ends were just pure-junk. Nothing but complaints when they first came out. I'd stay away, far, far away.
And it's a Crying Shame the XLR-V wasn't offered with a Manual. Those are slick cars you almost never see anymore.