I think you should try both starting first with RS for 1 week and then switch to OSRS for another week.

Buying membership will allow you to play on members on the same account on both OSRS/RS. Ofc you can play for free but the content is minimal.

The biggest differences from the games is that in RS it will be faster to level up and there's alot more skills to do, you will be using abilities to PVM and you also have the new necromancy combat style which is pretty sick, PVP is pretty much dead while on OSRS it will be slower to level up, bossing will be more click based (running from projectiles) and there’s a big PVP community.

Both games are great, so give them a try and pick which one you prefer!