Coming from an 06 Sti the driving and power delivery feel of the cars is very different. I cannot compare because it is not even in the same configuration.
RWD vs AWD, low set vs high set, V8 vs I4 turbo (your Evo), 2 differentials vs one etc.
I say, go and test drive one and see for yourself.
Now, if you ask me which one I had more fun with, I will say the STi but, the SS is whole new mature level of fun IMHO. Without an AWD system is like you can now control the car without those nannies and will most def. make you nervous when you hammer it but, that is where the fun starts.
If I could afford both I would def. do that but, got to have one beast at home
Bottom line. No regrets.
Good luck!