If You Would like the runescape you have always imagined in your head, play osrs, it's a bit dated but it's just getting better with OSRS gold a stupidly passionate dev team behind it that listens into the community and let's them decide what goes into the match, definitely go with osrs, it's a stupidly addictive clicking simulator which can trap you worse than the nastiest of drug customs
If You Would like another mmo with one hell of a background that could be described as wow with mtx plus a runescape theme, go with rs3, the Truth is they both have their places and audiences that enjoy them, however osrs is definitely the winner between the two for most people, hence why it conveys everywhere between 2-5x more players online at any given moment
Honestly if you've never tried , try them aside from the entire world and the lore and the set of abilities to train they've essentially evolved into two completely different games, osrs in my view is just way far less generic and ridiculously over the top, plus it's what most older players actually know, and also what they fell in love to start with.
I would vouch for one to try OSRS, in case you don't mind the grind, it has over 2x the playerbase and isn't infested with MTX, unlike Cheap Rs gold
Do not get me wrong, there's far more to being a good RS participant than with a large cashpile, and a lot of what actually counts can not be bought with gp, but using a sizable cashpile because you spent like $100 on MTX will surely give you an edge over players who do not buy MTX, irrespective of which variant of the game we're referring to.