You probably could have done a new thread / topic for this.
From what I found when I was looking is that 08+ has a 6spd auto for all power train setups previous was 5spd auto. If you want to make sure yours is fully loaded look for RPO codes under the spare tire cover in the trunk.
Look for:
1SG this adds lots of stuff can't think of them all right off the top of my head right now.
F55 for the advanced continuously variable Magnetic Ride Control with selectable ride preference
W21 for platinum model
JE5 for upgraded brakes
J56 for the best brakes (Brembo brakes from STS-V) - otherwise look for 6 lug wheels, non V brakes have 5 lugs.
TT6 for HIDs or look to see if there is a small rectangular cut piece in the front bumper between the headlights and grill as these are the headlight washers and are only found on factory HID equipped cars.
Things you don't have to see a code to know:
Speakers in the top of the front seats = Bose 15 speaker 5.1 Studio surround system, should have a huge Navigation screen too with a 6 disc in dash DVD changer.
Look for a seat and fan symbol on the HVAC controls paired with the heated seat button to get ventilated seats.
Look for a window in the dash above the instrument cluster as this is your heads up display (HUD).
The HUD gives you adaptive cruise control and driver awareness package.
Driver awareness is cruise that adjusts your speed base on the traffic in front of you the resumes your original speed after they are out of the way / speed up to at least your speed. This puts an active front facing radar system in the grill to monitor what's in front of you. This also adds side blind zone detection system that uses side facing radar in rear bumper to detect cars to each side of you. This also adds a front facing camera into the windshield to monitor road lane lines to help keep you from departing your lane unintentionally.
I'm sure some others will pipe in with other things.
I don't know of any changes to function / reliability for any model years newer than 08.
In my opinion the only reason to get newer than 08 would be for better chance of lower miles and better chance of still being under factory bumper to bumper warranty.
Speaking of that, power train is a 5yr 100k miles, bumper to bumper is 4yr 40 or 50k? Miles.
This is a must for all Cadillacs:
Get some form of service contract / extended warranty / get it certified from Cadillac as that gives a nice warranty boost.
Take your time and find the one you want. Test drive both RWD an AWD models. DO NOT BUY THE V6 model of any year unless you like issues with timing chains as when they break you need a new motor.