I would vouch for one to try OSRS gold, in case you do not mind the mill, it's over 2x the playerbase and is not infested with MTX, unlike RS3 Worth keeping in mind: while Jagex are unquestionably pretty aggressive in regards to pushing MTX on RS3, OSRS is not MTX free.
Don't get me wrong, there's far more to being a fantastic RS player than having a sizable cashpile, and a lot of what actually counts can not be bought with gp, but using a large cashpile since you spent like $100 on MTX will surely give you an advantage over players who don't purchase MTX, irrespective of which variant of this game we are talking about.
If you would like to grind endlessly osrs if you're searching for a more contemporary casual experience rs3vFor an MMO that I can just kind of Buy Rs gold drama to enjoy, I've always favored RS3. It cut back to the mill and pushed more of the high level stuff/combining items for greater efficiency and less work. That's part of the reason why some people today hate it: They really enjoy the grind. For grindy stuff, I've always favored Monster Hunter, where it's less amount of grind and more problem of mill.
There's no definitive reply to this - the best way will be likely to play for like a month per day and see for yourself, then pick 1 variant to continue if curious.