RS3, in my opinion, has better PvM content and similar to Lashiing's comment, is good for those who want to AFK train skills and get decent XP rates.
I think there's significantly more content and the quest lines in RS3 are severely under-rated - most people just space-bar through it but they have really deep and intricate lore.
The caveat to that is the MTX, however it never effected me. I didn't care about other people's wealth, skills, etc. I only cared about my own progress.
Further to that, the tick system on RS3 can be quite frustrating and clunky - but combat is much more rewarding and challenging.
OSRS is a mixed bag for me. I do enjoy the simplicity of it, however I do not enjoy the XP rates.
Both are in a decent enough state; but my bias for RS3 is because I'm grand-fathered in the $5 rate. If they cancelled and forced me to pay the current rates I would quit RS altogether and not come back. The game is not worth that much to me in my opinion.
that last price hike was pretty nans