Damn I'm 18 and thought I was doing good with my 9 year old Seville
I've driven both cars, and I prefer the ride and the interior of the STS-V. However, that car is alot more softly sprung than the CTS-V, and the exterior of the CTS-V is much sexier looking. It really comes down to what you want. Used STS-V's are in the low $30k's. The new CTS-V is up near
$60k, while used examples are right below $25k.
BTW, I wouldn't let my girlfriend tell me what kind of car to drive. If you like the Escalade, buy another one. Don't be whipped into buying a car she prefers.