I currently play LoL and plan on buying the full version of HoN once I get a second job (I was a beta tester).
As to which is better, I haven't personally figured that out yet which one I like more. However, I can say this:
Gameplay
In HoN, last hitting/denying means that you can shut down the carries in your lane by preventing them from farming up. The downside to this is that melee heroes or ones unable to harass/deal with harassment are at a severe disadvantage. LoL removed denying for this very reason. The pros and cons with removing denying in LoL is that on one hand, it means that tanks and non-carries don't have to worry about getting their farm. The downside is that carries are free to last hit and farm in a lane, becoming more powerful. In both games, you still have to last hit. Saying that "HoN takes more skill" is just retarded because you still have to have control over your champion in LoL in order to get gold from killing stuff.
As for heroes, HoN's heroes had some serious balance issues when the beta was over (I'm looking at you, Zephyr). While I don't know how the current state of things are now that the beta is over, but I imagine it hasn't changed much. There aren't really any champions in LoL that can kill multiple opponents alone unless they have been severely fed (which is true in DotA, LoL, and HoN). However, in HoN, it's not uncommon for a hard-carry to tear apart entire teams once they get a specific item. Currently, the two most overpowered champions in LoL are Soraka and Heimer, both of which are support champions, who require decent teammates to be broken. One thing I like about LoL is how team composition is the key to countering champions, whereas HoN specific heroes counter other heroes (Vindicator vs. Tempest, for example), making LoL even more team-oriented.
For spells and abilities, HoN takes the cake. LoL is currently all about harass and kiting, backing it up with healing. This makes it incredibly difficult to initiate, leading to 10 people staring at each other next to a turret. In HoN, the maps are bigger with more areas to gank. Combine this with some quick nukes, and it makes it much easier to start a teamfight when it's 4v5 or 3v5 to begin with. Not to mention, there's a longer distance in between each tower, making it easier to chase. HoN also has much better stuns and crowd control, whereas LoL lacks champions able to set-up ganks or teamfights, especially with the summoner spell, cleanse. While LoL may seem quicker and more fast-paced with spamable spells, the reality is that the game oftentimes feels stagnant with neither side able to deal any damage to the other because they are too nervous to move in.
I prefer HoN's items, especially the bottle, the portkey, and the powersupply. These three items would fix a lot of the problems currently in LoL that I mentioned above. Portkey allows for teamfights and ganks to happen anywhere, powersupply helps you kite or chase or push and negates the effects of spamming spells, and bottle allows you to do the effects of both to a lesser extent, especially if you have a powerful rune. A few of the regular items in LoL seem redundant or imbalanced, such as stark's vs. malady. That's a minor problem, however, since both games have a unique variety of items to chose from with none of them being entirely game-breaking.
Gameplay Summary: HoN is centered more around carries and protecting your team's carry. LoL is more about the team as a whole. LoL can also be stagnant at times due to having very few nukes and stuns and more harassment and healing spells. HoN has some balance issues with its heroes, considering some of them simply able to just roflwtfpubstomp entire 5-man teams all by themselves.
Technical
Graphics wise, I like both. LoL stays true to DotA by imitating the cartoon style of the WarCraft 3 engine. HoN has a much darker direction in its design. It's almost like people saying that "Diablo 2 has better graphics than WarCraft 3" or vice versa, you can't really compare the two.
HoN has better servers, hands down, no questions asked. I can play when I want. LoL's servers are constantly down for maintenance and updates or some stupid error in their system to the point where I am unsure if I want to invest money into the game.
HoN has in-game voice chat. LoL does not.
HoN has better matchmaking, especially considering that psr and msr of players is public information. When I was beta testing, I really enjoyed the 1600+ level of matchmaking. Anything below that generally had someone feeding or bad sportsmanship, but at 1600 and above, players knew what they were doing. As stated before, level 30 means nothing in LoL simply because you could have gotten there by losing and feeding every game you have ever played. I also noticed that when people are level 30 with more games than me, they get a "holier than thou" attitude, oftentimes blaming the team for everything. In HoN, if there was a 1795 saying I was a noob, I knew I had better get my shit together because the guy playing our carry with 1795 msr knew what he was talking about and I should take his advice. In LoL, I have no idea if I should listen to that lvl 30 with 500 wins because he may have lost 1200 games, meaning I have a better win/loss. There is also the issue of solo queuing and you end up playing with 4 other guys who also solo queue'd, and the other team is a 5-man pre-made pub-stomp squad. This is extremely frustrating as my team is trying to figure out what everyone's strengths and weaknesses are as players in the first 5 minutes, while the other team is systematically destroying us during their 25th. game together.
HoN also takes the cake on stat tracking. You can get replays, player statistics ranging from number of towers destroyed to gold accumulated, and detailed information about every match played. LoL only shows the most recently played champions and stats on the most recent 10 games, with no replays available or any indication of what the other players in those games were like.
Lastly, HoN handles leavers and punishes leavers more effectively than LoL. This is also a point where it helps that HoN is more carry-oriented. If your carry is strong enough, your 5th. player may not have been needed. In LoL, being gone for even 5 minutes means that you will probably lose that match, as you cannot win any teamfights or push unless you have an equal number of players. Furthermore, once a player has left, there are no benefits to having that person there, they just sit in town, whereas HoN distributes that player's items and gold to their teammates and allows control of the hero so they can do simple tasks, such as defending a lane against a large creep wave.
Technical summary: HoN takes the cake. Graphics-wise, you can't really compare them. But stat-tracking, leavers, in-game voice chat, server issues, and matchmaking, HoN is much more reliable and S2 games did a great job of handling these issues.
Spark's notes version of this post:
Gameplay wise, they both have their merits. With technical issues, HoN is superior.