extended range is always the first advantage that comes to mind when considering 5+ strings on a bass guitar, for example i play in a band where all the guitars are tuned half step down so i'd have some problem with just a 4 strings, then sometimes they drop do Db and i found it very difficult to do on a 4 string, then again, i'm used to play on a sevenstring guitar, so it doesn't bother me to play on a larger neck, and i'm using an ibanez sr505 so it's not so big.
one of the things that you'll have to keep your eyes on if you get a 5 string
is that the 5th string sound crappy on some basses, so 35" scale is a good bet
my bass is 34" scale and i'm very happy about the way it plays in the low registers, it has a mahogany body, and it's a versatile sound, i can get from middy rock sounds, to scooped out newishmetal sound easily with the onboard eq
so, for me 5 is right :P ( 6 is great if you miss guitar
)