For me, it was the ol' Billy Sheehan 'If I'm going to confuse myself with extra strings ... it might as well be six.'
As of recently, I've sworn off 6'ers (not sure how long it'll last ... they are a bit addictive

So much so that I even moved onto a GT-7, then realized I didn't like the ring of a high F#). The main reason for my recent reluctance to play 6 is; I've finally found a 5'ver that I'm just as comfortable with, as a 4-banger. Prior to my first 6'er purchase, I never found a 5'ver that I was "at home" with. I even tried different string spacings. The odd thing is ... I'm more comfortable playing a 6'er than a majority of 5'vers I've played ... even with a narrower string spacing than I perfer on a 5. Add to that, I don't do as much tapping and chording as I used to (... for a while, I as all about it).
As -pointbass- pointed out

... there is a different approach(s) to playing a 6'er. Personally, I find myself playing with my first three fingers most of the time ... using my pinky to mute (as I've always done, no matter the number of strings). I never really had a problem with slap'n in tight spaces ... however I find that <16.5mm is a bit too tight for my slap'n taste. Also, I wear my bass higher (up towards my chest); generally closer to my stomach with lesser strings. As well as other things that I don't even think about ... I just do.
IMO ... if you're wanting to explore the wonderful world of six strings ... get yourself an affordable sixer, with as close to your string spacing likes ... experiment with it ... and have fun. Frankly, I believe it'll make you a better player in the end.
(I would suggest a Peavey TL-6 ... a fantastic workhorse of a 6'er for pretty cheap)