I have no experience with the 506 but I can make a strong case for the 510. I bought it new back in the mid or late '70s, used (and maybe abused) it frequently, dragged it around with me over the decades from apartment to apartment and house to house, and (I'm embarrassed to say) NEVER serviced it, lubricated it or took care of it as I should have. Guess what? After nearly 35 years, that marvel of German engineering still performs like it did the day I got it...with perhaps the ONLY exception being RCA plugs which are starting to go bad (if I grasp the back of the plugs where the cables enter and wiggle them, I can induce a hum).
If I have any regrets at all it's that I didn't outfit it with one of the nicer optional REAL wood bases which were available at the time, but even with the plastic faux wood-grain base, the 510 turned out to be one of the smartest purchases I've ever made!
BTW, it came new with a Pickering XSV/3000 cartridge (dealer option?) which I changed out only last year for an Audio Technica AT-15XE cartridge body and ATN-20SS Shibata stylus. It all works and sounds great!
Good luck with your decision!