Depends.
RS is more simple, play when you want and go at your own pace.
WoW you get hooked into playing and if you actually go hard you'll have set times for raids, PvP events which you'll have to commit to. Once you get to max level WoW is really fun, but the grinding is pretty bad. Although with RS you sit in one spot for hours clicking a tree to level up woodcutting.. so.. both are pretty bad.
tl;dr - WoW if you can commit, RS if not.