uh...
for ss: lighter, dependable, quiet, simple. easier to clean, easier to keep clean, once set up properly no finicky adjustments to make, easier to set up.
oh, and rear wheels with no dish are quite a bit stronger.
for fixed, all that and the constant connection to the road, the flywheel effect, the beauty of a true trackstand, and the jedi-like extension of awareness that you get from having to plan ahead.
and then there's the ride itself...but like all soul-riding, there's no point talking about it.