hi,
by way of introduction, i played 4-strings between the ages of 14 and 29; then went to 6-strings exclusively. been at it for almost 35 years (old!).
as @nostatic mentioned, i advocate just going for it on 6. it wasn't an original idea. when i first decided to take up the 6, i read anthony jackson saying he had stopped playing 4-string basses (don't know if he did for all his session work or not).
seemed like a good idea to me, so i went for it.
potential drawbacks: at first i had the occasional mishap of reaching for a low G and hitting a low D. that went away pretty quickly. other musicians (and bass players) might give you shade; there's a whole mis-perception of why people play 6-string bass. you *might* lose work based on this perception and on *how the bass looks*. people hear with their eyes . . . (the corollary to this is play the music, not the shiny toy)
benefits: more possibilities for expression. timbre, range, polyphony, etc. it's a great tool for making the music you want to hear. there's a lifetime in there, if you want it.
advice: get a good instrument. a bad instrument is not gonna inspire or challenge you. when i started there weren't a lot of choices. i was playing fenders and got something pretty close and have been happy with that (i still have my first 6-string bass and use it all the time). so . . . if you like 34-inch scale, 19mm spacing keep with that; if you like something else, keep with that. if you don't like active, get a passive 6-string (or vice-versa). my general thing is that i want to feel at ease with what i play—i don't want to think about the bass, i want to play music on it.
dunno if that helps or not.