Just some things that aren't noted already
RS3 may have an updated combat system, but you can automate all of your skills or a selection of your skills so you don't have to do everything yourself.
RS3 also gives you the options to play in legacy mode to give you a legacy combat feel, gives you options for a legacy interface, and gives you options for quality of graphics.
We don't get updates as frequently, but that's because when you have so many options, you have to create many different levels of the same thing, ranging from highest to lowest quality.
OSRS lacks these options.
Because the options are quite limited, and the quality is fairly low, it's much easier to slap some quickly whipped up graphics on a concept and implement it.
OSRS is, or rather was, mainly for nostalgia, along with a fresher and more profitable market.
If you're mostly interested in making gold to sell for fg, OSRS may be of more interest to you, simply due to the rates.
OSRS has also deviated far from the original game that it was rebuilt from, and in my opinion, no longer has the same nostalgia it once offered.
Regardless of what game you decide to go with, they are both quite different from what they offer.
Don't let someone else choose for you though. Try both, and go with whatever you prefer.