If you want the runescape you've always pictured in your head, play osrs, it's Somewhat outdated but it's just getting better with a stupidly passionate dev team supporting it listens to the community and let us them pick what goes into the OSRS gold match, definitely go with osrs, it is a stupidly addictive clicking simulator that will trap you worse than the nastiest of drug habits
If You Would like a different 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 who enjoy them, but osrs is definitely the winner between the two for most people, hence why it carries everywhere between 2-5x more players online at any given time
Honestly in the event that you've never tried , try them aside from the entire world and the lore and the set of abilities to train they have essentially evolved into two entirely different games, osrs in my opinion is just way far less generic and ridiculously over the top, plus it is what most older players really know, and also what they fell in love to start with.
I'd vouch for you to attempt buy runescape mobile gold, if you do not mind the grind, it's over 2x the playerbase and isn't infested with MTX, unlike RS3
Don't get me wrong, there is far more to being a good RS participant than having a large cashpile, and a lot of what really counts can't be bought with gp, but having a large cashpile because you spent like $100 on MTX will definitely give you an edge over players who do not purchase MTX, regardless of which variant of the game we are talking about.