Well, all three are fairly similar so when it comes down to preference, it's usually a somewhat fine line you divide the three by. These are the reasons that I prefer LoL though.
1) I hate creep denying. To me, it adds needless levels of time and complexity to the game. You are forced to pay attention to more variables and by denying farm, the game length can increase considerably as long as players are competent and not dying early.
2) I really enjoy the more vibrant art style that LoL has. The champions just feel more unique to me and I thought HoN had a darker, more realistic art style that I didn't care for in this kind of game.
3) The LoL community is not as bad as DoTA or HoN. This is completely anecdotal, but I've had much worse experiences with those two games than with LoL. Don't get me wrong though, you need tough skin for any one of these three games and you will be thrashed for playing poorly in a ranked or even regular game. I do enjoy they offer vs AI games though so you can practice a new champion.
4) I havn't played HoN or DoTA in a while, but currently LoL is quite balanced barring unequal player skill. Many champions have their good points and bad ones, and while you have your top tier and low tier champions, these differences don't really come to light until much higher ranks in the game. Most of those champions considered top tier right now are about to get a round of nerfs though so no champion should really be too crazy.
If you do get involved with LoL, try out the free weekly rotations of champions until you find a couple you like. You progressively earn IP after every game and you can use that instead of real money to buy champions, so use this to buy the few champions you enjoy. Money is only useful to buy stuff faster, or to buy skins.