I was planning on pre-ordering one of them as I only have enough money for one new 3DS game right now. I absolutely loved the first Luigi's mansion but I also played the Fire Emblem: Awakening demo and thought it was fantastic so far. I know this is more preference but leave your opinons and why you formed them.

All the best,
-Nick

probably luigis mansion i really want to see the multiplayer... if there is one

I prefer Luigi's Mansion Dark Moon. Loved the original game, really like the look of this one. And as you may have guessed, I'm just more of a Mario fan in general.

I would recommend that one, but honestly, I don't know which game you'd enjoy more.

Fire Emblem will probably offer way more content. And it comes out earlier. And if you want a physical copy, it's likely you won't be able to find one after release, since Fire Emblem games can get quite rare.
I'd go with Fire Emblem.

Now, you'd probably be better at judging which you would like more since they're two totally different games.

Luigi's Mansion. More my type of game, I won't be terrible at it.

You liked the first Luigi's Mansion, so there's that. I on the other hand thought it was incredibly boring (one big Easter egg hunt). If I get Dark Moon it will be at a discount for sure. I'm getting Fire Emblem Awakening myself. It looks like the best in the series so far. I'll know for sure soon enough.

It takes a while for a FE game to become rare, he'd be able to get it fairly easily for the next two years either through stores or online retailers.

Anyway, FE typically offers way more playtime than a game like luigi's mansion; this one in particular. That would be the deciding factor for me if you want both equally now.

depending on what type of game you like

Fire Emblem and I don't care what type of game you prefer

I haven't played the first Luigi's Mansion and love the Fire Emblem series so I would definitely go with Fire Emblem first! Also, in my opinion:
1) FE will give you more value for your money.
2) You've tried the FE demo and liked it but there's no demo yet for Dark Moon.
3) Between these two, I think that Dark Moon is the one which will see a price drop first, so you can buy it later and save some money.
With that said, I hope you're happy with whatever choice you make!

Luigi's Mansion 2, to support an actually good game.

Don't listen to Mickeymac, i'd go with Fire Emblem. It's undoubtedly longer, and would give you a new experience. You could probably hold off on LM: DM for a month or two with Fire Emblem.

How can you say that Fire Emblem: Awakening is a bad game unless you have played the demo? Then again, demos can misportray the full game sometimes. I would say either one would be fine to be honest. Although, Fire Emblem: Awakening will probably be the longer game so...

@nrose619 To quote @theblackdragon, "For all those having trouble choosing between 3DS games, this is the thread for you! Ask for recommendations here." Please and thank you. :3

I don't care about Fire Emblem so I pick Luigi's Mansion

It's a tough call. I don't know why but I want Luigi's more right now. Nevertheless, Fire Emblem will be a great game (probably better than Luigi's) and much longer and very importantly, it will have free dlc the first days after launch but will turn to paid dlc soon after.

Also, if you live in the US, there will be plenty of time between them.

If you need any more help deciding which 3DS title(s) to purchase, please feel free to ask here. Thank you! :3