Hard to compare really. WC3 definitely has a higher skill cap because there is so much macro involved, as well as microing a lot of units at once. You need an insane amount of awareness to play at a hgh level, not to mention some ridiculous muscle memory and twitch skills to micro an entire army.
LoL seems more basic, but timing is crucial, as is decision making. If you use an ability at a wrong time your entire team can get wiped out because of it. There's also the complexity of the last hit battle during laning, and the fact that you need to coordinate with 4 other people.
Both require a lot of skill to play at an elite level, but WC3 probably takes a bit more.
I find both to be interesting, I think it just comes down to whether or not you prefer playing alone or on a team.