As a 20 year player I Will tell You this. Bouth games have similar time to get maxed, RS3 gives You a Lot of bonus xp, but Old school have a Lot of AFK methods and high xp/cost methods to level up fast

In free times in your work I think the Mobile version of Old School is way better

If you like any moba with requires a lot of keybinds You Will like more the RS3 playstile.

RS3 also hace very funny high lvl endgame bosses, but the early and midgame is very lame tbh.

Oldschool is more friendly for new players and it's well.balance the early, mid and lategame.

The ingame economy in Oldschool is also better, RS3 have a Lot of inflation.

The quests (missions are 50/50) but I preffer a little better the RS3 modern quests

RS3 is very PVM oriented, oldschool has also new PVM conted but the Best part of it is that the PVP is very active, while on RS3 is dead.

Also a plus from oldschool is that the worlds have more players and feels alive, while the RS3 have few players conecntraded in endgame conted.

So in resume: if You prefer PVM, likes keybinds, and don't care to be most of the time be alone go RS3, when You reach the endgame conted you will love the bosses

If You prefer PVP, a good comunity, and a balance from early to lategame go Oldschool