Both games Are Nice.
You can easily afk combats in rs3 like you can in osrs if you follow What Xrmd said.
Its first doing endgame bosses etc you need to learn How to do it really.

Also the xp rates in rs3 is quite good compared to osrs. So you Will see people Are gaining for 120 skills and 200m skills instead of 99s like in osrs.

Gl