Both are good games, but LoL is a lot more casual than DOTA 2. You can just jump into LoL and have a decent amount of fun. Not so with Dota 2. It's more like a decision to decide to learn to play, an investment of time. In the end, it will be more rewarding, but it takes work to get there. Checkers Vs. Chess, IMO. I have a few hundred hours in LoL, and although I'm in the Dota 2 beta, I'm still debating whether I want to make the time commitment to learn the game.
Another major plus to Dota 2 is that everyone has access to all heroes at full power, all the time. LoL you're limited by the current hero pool, trait lines, etc.
And anyone who thinks Valve is too late to the party... ha. Dota 2 is consistently at the top of Steam game stats, and it's not even released yet! Not everyone who wants to play can play.http://store.steampowered.com/stats/
Currently 100k playing right now, about as much as Borderlands 2 and XCom combined.