Expanding on the above, slightly: the sight and terrain mechanics work differently in each. There's a night/day cycle in HoN that affects visual range as well (not sure about LoL).
LoL in turn, has a persistence mechanic in that your summoner (your account) levels by participating in matches and then spends those points to unlock characters. The points to unlock characters can also be purchased. IN HoN, nothing persists save your stats and all characters are unlocked from the get-go.
And on a purely personal note, I prefer the art direction of HoN to LoL. But since they're both free to try atm, I'd really recommend downloading both and seeing which one works for you.
