I have a 60's and it' thin. I think the 50's is fatter. In my opinion, if you want a "faster" neck, so to speak, go thin. If you want one that's more ergonomic, and you are playing more complicated chords, the fatter neck won't wear your hand out as quickly.
Just remember that one '50's neck profile' isn't necessarily the same as another. Yes, 50's profiles tend to be thicker, and 60's profile tends to be thinner, but there is a LOT of variation. Consider the 57 goldtop reissue. The neck on that is a MONSTER!! Classic baseball bat. The R8 is a bit smaller,. but still chunky, and the R9 a bit smaller again. Similar with the 60's proifile. Some tend to be thicker or thinner than others, so you may play one 60's profile and love the feel of it, and then play another and not be too impressed.
It depends. The neck on my '00 Les Paul '59 RI is perfect...but the 50's neck on my buddy's ES-335 Historic haa big shoulders and needed to be rounded a bit more, IMO. Generally, I like the 50's necks as long as they're round enough. Lew
I've got the 50's neck on my Les paul standard, and i love it. Not HUGE, like my freinds '56 goldtop, but nice and full, and it really seems to fill up my hand. I like thicker necks in general, though, so....
If you're shopping for a les paul, go play some, find out for yourself...