a fiver will open up greater chord/arpeggio options, as you can (for example) play low G on the B string (8th fret( and 'B' at 9th fret on the D - if you're into this style of playing then a five string is a no-brainer.
Depends on your style of music though, as sometimes an E string dropped to a low 'd' sounds better (in a heavy rock setting) than 3rd fret on the low B.