The CTS V1 is "only" 400 HP while I understand the STS V is 469 HP.
V1 is a stick which some people object to for city driving - I find the stick to be more fun.
STS V is auto. Much more laid back driving.
I've never driven a STS V or XLR V with the blown Northstar. Anybody in OKC area want to loan me one for a test drive?
There are lots of aftermarket parts for LS6/LS2 in the V1, so HP limit is how fat your wallet is.
I'm not sure what's available for the STS V.
Might want to research the axle on the V1 first before you commit though..