I've done both. I have to say that Spring Break is way more crowded than President's Week. Here are my comparisions:
PW: - shorter hours during the week (DL: 9-8, DCA: 10-6)
- FPs almost didn't run out (except Space which was new last year)
- Lines for major rides around 45 minutes (Indy, Splash, Soarin, Screamin)
- entertainment is limited
- easier to manuever through crowds
-weather is a little cool
SB: -longer hours during the week (DL: 8-12, DCA: 10-10)
- FPs ran out for popular rides (Indy, Splash, Soarin, Screamin)
- Lines for average rides were around 45 minutes (Roger Rabbit, BLAB, Autopia, GRR, Sun Wheel). Lines for popular rides were 60+ minutes (Space and ToT weren't open yet last time I went)
- entertainment is on full
- crowds are almost wall to wall
- warmer weather