Well it depends. I went directly to the 6 string from a 4 string just because I wanted so much to have these extra notes available. But I think it also fits my playing style very well (I love playing arpeggios and lots of scales and fast runs, tap etc. as opposed to (e.g.) slap & pop). So that's the first question you have to ask yourself, if it doesn't fit your style then it's probably not worth getting one. If you don't want to play solos or use higher harmonics, tap chords and stuff like that, I don't see why you would get a 6 string instead of a 5.
If you want it just for the fun of it, then it doesn't matter, just get whatever feels more comfortable. Actually, that's the best suggestion I have: just go out there and try basses. You might find that 6 string basses are too big and nasty and that you don't need that high C string after all!
If you're wondering about how difficult the transition is etc., I can assure you it's not hard at all, it just takes time & practice. Don't get disappointed just because you can't do the same things you were doing on the 4 string bass straight away.
Hope I helped, have fun!