dont know about the cts but stay away from the sts. if you do get one buy a new one with extended warranty.
the one i HAD was in the shop 4 times in the first 2 months i had it. then 3 months later it went back to the shop for a repair that cadillac wouldnt warranty. called cadillac in detroit and they said they would knock off $200 off a $850 repair, for the trouble i'd had with the car(thanks a lot)
then it was ok for about a year. next thing i know its in limp mode. engine and tranny ecu's went bad(out of warranty to boot). $1500 later and i traded it in the next day. i'll never own another one.
it was all electronic stuff that went bad. except the gas tank. yeah, the gas tank had to be replaced. took chevy 2 weeks to get it.
NIGHTMARE!