HoN is more strategic and skill-dependent whereas LoL is more action-packed and casual. The learning curve in HoN is a bit more steep than it already is in LoL, and you will probably never know every damage type, manacost or cooldown time. Ofc both games aren't real strategy games and both need a big amount of skills and knowledge.
Although LoL claims to be free, you have to play hundreds of games to unlock every hero. In HoN you can play every Hero if you buy the game and when you start your first game you have the same chance to win like everyone else (no runes or masteries).
HoN also has replays, addons and big cash tournaments. Oh and your eyes won't start to hurt after a few hours of playing ;)