So old school is that enjoy RS gold a game stage that is more simplistic and it's also the way that it was in 2007. Rs3 has more supervisors to kill and far more content to not being restricted on what material is released. And it existed for over 15 years with upgrades. In case you have any additional questions feel free to message me here or if you see me in game my rsn is Ldking.
Apart from nostalgia, another reason why some people can prefer OSRS is it rs3 is packed with microtransactions--and moreover, RuneScape is quite invasive about it, throwing up ads on your face in login, and projecting in an MTX popup every day you cannot disable. The MTX promotions also indirectly promote xp into the players, and generally give out free xp for logging in every day, which to an extent, cheapens the value of rs3's acccomplishments.
OSRS nevertheless has the capability to purchase in-game gold through MTXs in the form of bonds, but they definitely don't irritate the players from attempting to push MTX in your face as much as you can. That having been said, if you don't care about MTX (an increasing majority of games seem to have them nowadays), then rs3 is generally a much better game--it has far more content, more contemporary images, a combat system that arguably fits pvm better, and now they've just released a brand new RuneScape skill, so there is a great deal of action right now as well. In that respect, you'll have to create your own choice.
For PVM, I actually can't think of any MMO that does it much better than RS3, if you don't especially like raid-related content. For PVP, it is dead in RS3 out of warbands, and while OSRS PVP is somewhat alive (wilderness generally is fairly dead), it's VERY unique and not something a new player would like. OSRS thrives on nostalgia, but depending on your cheap RuneScape gold wants, I would say RS3 would be preferable and generally quite enjoyable for you.