Stock vs stock i'd take the GTR.
Yes the new commodore's have more power and torque however they are also listed at 1700+ KG so 1800+ with driver. That means the power to weight ratio's are similar (6.94 GTR vs 6.3 SS). As the SS is RWD with a lot of torque, it will be much harder to get off the line and up to speed where as the 32 GTR can dial up 5K, sidestep the clutch and disapear without much of a fuss.
The SS will have a little bit more top end but it's got to have the right stretch of road or track to even utilise that advantage and then it will still have to pull back the gap the GTR has gained from getting off the line.
I had a wildly modified soarer in the past that made nearly 400rwkw and it would get slaughtered by GTR's all the time because it didn't have the room required to wind up. Mind you if it did get into 3rd gear on boost it'd be a different story, but it proved to me the point about having usable power.