My suspicion is the the SS would perform better in an autocross environment. I haven't driven an SS, but the feel and ability to supply some oversteer with a RWD car seems like a substantial advantage.
When I've pushed my Fusion Sport, it's quite evident that it's a heavy FWD biased car. I've also had a few incidents where the car doesn't behave as expected on hard acceleration from a stop or slow roll while making a hard turn - IE beating oncoming traffic onto a highway on ramp or at a traffic light. It's certainly capable of putting down power a lot more effectively than my 14 Ti, but there are limitations.
I'd probably choose the SS if snow and price weren't really factors in the buying decision.
2017 Fusion Sport in lightning blue
401A, moonroof, cooled seats, navigation, and driver assist
3M Color Stable 35 tint, Drive Bright LED puddle lamps; TTR FMIC and Unleashed 93 tune