There are several versions of the 506, but only one 508. The primary difference between the two is the metal plinth on the 506 (either version) and the integral molded base of the 508.
However:
If the 506 you're looking at has an Ortofon, then that's likely what you're stuck with, because you can't change cartridges without an adapter that's next to impossible to find. The 508, OTOH, likely already has the adapter because it's got an Audio-Technica...which means that you can change the cartridge if you want, and also new styli are more readily available.
I've got a 508 - with the adapter - and I've enjoyed it for over ten years. The 506 has that traditional Dual look some like, but I like the look of my 508.
Just my opinion, YMMV, Chacun a son gout (or the equivalent in German)