Tldr; RS3 and OSRS are both worth giving a shot.

For some perspective, I have played OSRS from 2014-present and have 2k+ total on my main, and I started RS3 in May and I am already 2300+ total.

I actually get more nostalgia from RS3 because it has so many years of content to catch up on. OSRS has had a lot of updates since its release, so I wouldn't count on much nostalgia besides the very early game. The updates on OSRS, for the most part, have been good though, in my opinion.

I don't really like RS3 combat, and use legacy mode which makes it reminiscent of OSRS combat, but I enjoy skilling more on RS3 than on OSRS in most regards.

I would recommend having a decent computer (I don't think it has to be too crazy, mine isn't) if you do play RS3 because it looks pretty nice on max settings.

If you want to use mobile, I find that the OSRS one has a better layout, plays less clunky, and suits the simple nature of the game more in general.

Making money on RS3 seems more approachable to new players, and from what I have seen there are more ways to make money in general. The economy on RS3 is inflated, but it doesn't really become an issue until you get to max gear and rares. OSRS it can be hard to make money besides grinding the few meta options.

Hope it helps if you decide to read my long winded reply.