Gary is out of office, but I'll chime in on his behalf. Both are very good tires. The PSS has a bit more grip, and very responsive handling. The S04 has very stable, confidence-inspiring handling, so even though it is slightly behind in lap times on the track, it is very rewarding to drive. You will find people who like both.
If you are reluctant to change sizes, the S04 is an excellent choice.
I have accepted a new position at Tire Rack and am no longer in the sales department. firstname.lastname@example.org
is available to assist with M3 questions.