I have both, and they are not the same car
But with GXP values the SS is a great car for not much more money
You want a 2015+ for the MR suspension and visually - the rear brembos, first year of the manual, and different rear valance panel [14 was textured]
The 16/17 have the updated front clip, hood vents, bimodal, and the wheel change
If you want an auto, just like the GXP, expect to pay less.
But I enjoy driving my wifes A6 SS as much as my M6 SS anyway.
2014 Chevy Cruze
2013 Chevy Avalanche
2009 Pontiac G8
2009 Saab 9-3 Combi
1988 Volvo 740