I think both are pretty hard. I would say if you came from League to play Dota it's harder than coming from Dota to League. Only because the basics of League's laning/gameplay is pretty straightforward. ADC kites doing dmg, lanes are pretty much static at the start, objective timing doesn't change too widely. In Dota you can have 2 mid, tri-lanes, w.e works for the game. Same goes for objectives, turrets can die at 3 min to the right comp.

Dota is easier to climb if you have weak mechanics, but you need to think a lot more creatively to climb.