Most of the time I've worked with S-glass, and it is much more difficult to work with around complex curves and stuff, but the benefit sets in with better structural rigidity. Not necessary, but I've got 18yards of the S-glass bid in the shop right now, and all I'm doing with it right now is making a foam-core box... yeah overkill.
Conclusion, if you don't mind the extra work and difficulty, s-glass is better. If you just want to get through your fiberglass work and on to flying, e-glass is perfectly acceptable.