Well, Mabinogi is restricted to the USA(+Canada?) and Oceanic regions - I was actually asking about your state. Here's a short review/comparison of the two games. Read it if you like.
Both games are potentially very grindy. You'll notice this over the course of Maple Story, and certain skills in Mabinogi are EXTREMELY tedious to train (I'm experiencing this currently)...
I have multiple characters at fourth job on Maple Story but I'm still having trouble getting rank 1 Windmill in Mabinogi.
Definitely go with Maple Story if you have a bad PC or connection. Mabinogi is extremely skill-based and requires good timing. Unfortunately it also a lot of lag issues. If you're really far from the servers in California, I really wouldn't bother. Especially if you want to be a ranged user (a lot of issues with ranged attacks and lag, such as freezing/being unable to shoot while the monster continues to attack). You might also not be able to do more advanced techniques which involve quickly using various skills as combos.
Technical issues aside, Mabinogi is a much deeper game. Maple Story can be a lot of fun, and it's definitely up there among the best F2P MMORPGs - both of the games you listed are - but its endgame is very shallow.
I noticed that the grind can be very difficult and the rewards are very small in the end. In comparison to other games I have played, it doesn't have a lot for high level players to do, and if you put a lot of time into it, you might be disappointed. As a level 14x Bishop, all I really have to do is Horntail, Zakum, and a couple of Party Quests... or level up more. I've already maxed out all of my really 'useful' skills.
It seriously makes me question how people get to level 200 - sure, it's not THAT difficult anymore... but how do you motivate yourself? When you get there, you basically have to start a new character or quit the game. There's really nothing left for you to do. I would inevitably drop into a deep state of depression.
Mabinogi is much deeper and if there's an end to the game, you probably won't reach it in your lifetime. Your character continues to grow and get stronger and there's no set limits. Not even classes. You can be a giant wielding a huge *** hammer and still pull out a wand and cast magic.
Or you can spend your time in towns, talking to people and levelling up various life skills like tailoring, weaving, blacksmithing, or composing/playing your music. The cool thing about life skills is that they permanently increase your base stats - so levelling them up also makes you stronger when you do decide to go into battle. Plus, Mabinogi has a storyline to keep you busy which is actually pretty cool, and there's a lot of interesting activities, too.
Overall, Mabinogi is the better game, but its technical flaws are vast.
Playing as an elf from New York, even with a high speed connection and good PC often makes me want to ragequit (the elves are especially buggy). Maple Story is a great game, too, and getting new skills and levelling up can be just as fulfilling as it is in Mabinogi... but don't take it too seriously.
As for cash shop, Mabinogi will cost a crapton if you want to be 'pro'. People will argue otherwise (and the situation has admittedly improved with free rebirths) but paying for mounts/pets/advanced services/(especially)weekly rebirths/etc. will help you out and speed up your progress by a LOT.
Maple Story is famous for having one of the least overpowered cash shops... there is a couple things now that give more of an advantage to the player, but most of it is cosmetic. If you're one of the few OnRPGers who does pay for cash shops, I actually like Maple Story's more, though. I like being able to change my character's looks a lot. There's tons of mounts, clothes, weapons, accessories, pets, pet accessories (pet clothes and auto-pick up/potions), whatever. None of it adds stats (it just covers your actual equipment) but it really makes you feel like your character is unique and it's tons of fun to pick all your stuff out and make your character awesome. So, whether you pay or don't, Maple Story comes out on top in the cash/premium department.
Community sucks for both in general (they seem to pretty much have the same community - Mabinogi has more weaboos) but there's a few intelligent people. Mabinogi seems to have more intelligent people, but the majority still remind you of that angry german kid banging his PC from YouTube.
As he said, Mabinogi is a much deeper experience with storylines. The combat is frustrating as even a .5 second lag can kill you. If you don't pay up for a mount life will be very difficult for you and advanced services make the game way way easier.
It has -great- music and a nice artstyle. There's a real sense of adventure here but imo the dungeon system kind of kills that mood.
The world is pretty barren, especially on the new continent.
On Maplestory you pretty much just kill mobs and do killing or collection quests. There's nothing awful in the cashshop. You can even get 2exp from the family system now. The only thing to look forward to is 4th job. After that you just...kill more mobs and hang out with your guild/friends/etc.