If you were thinking of buying a 3DS XL to play either Luigi's Mansion: Dark Moon or Pokemon Mystery Dungeon, Nintendo is offering you a little extra incentive.

If you register a 3DS XL on Club Nintendo starting today, and register one of those two games, between March 21 and April 30, you'll get a free downloadable copy of Super Mario 3D Land, Professor Layton and the Miracle Mask, Art Academy: Lessons for Everyone!, Star Fox 64 3D, or Freakyforms Deluxe: Your Creations, Alive! Your choice, so you aren't stuck with Freakyforms.

http://www.joystiq.com/2013/02/14/buy-a-3ds-xl-and-luigis-mansion-or-pokemon-mystery-dungeon-get/

Kid Icarus: Uprising
Etrian Odyssey
Fire Emblem
Dillons Rolling Western
Freakyforms Deluxe (really, it's a great game although it looks strange)
Denpa Men
Heroes of Ruin
Pullblox and Fallblox (named Pushmo and Crashmo in the US)
Bravely Default (it have to come to the west)
Code of Princess
Project X-Zone
Pokemon X/Y
Hana Samurai (heard it is named Sakura Samurai elsewhere)
Crimson Shroud
Liberation Maiden
Luigis Mansion: Dark Moon

 Technically Monster Hunter (also WiiU) and Zelda:OoT (N64) are not exclusive. While I prefer MH on home console I recommend Zelda for 3DS. Edit: i see you own no WiiU, so you should get MH-greatness on the 3DS.
The excellent Resident Evil:Revelations will come to home consoles, I recommend to get it there.

Edit: The XL is worth it. In europe it comes without the power cable though, that must be purchased separately or taken one from DSi or DSiXL (they are the same).

I'm undecided if I should take advantage of this 'deal' or just wait till Black Friday. I'll probably just get it Black Friday. Hopefully there will be deals on the games I want. I bought Pokemon White 2 last week for $15, and I constantly check the price of the 3DS XL and other games I listed that are out. If I see the XL for cheap I'll just snatch it up and get Luigi. If not, then Black Friday it is.

Super Mario 3D Land - Well executed
Professor Layton and the Mask of Miracle - Amazing game for Puzzle fans
Ocarina of Time 3D - Great Remake
Theatrythm Final Fantasy - Time Sinker

Games I have been keeping an eye on -

Castlevania: Mirror of Fate - One of my most hyped games for the 3DS
Fire Emblem Awakening - Seems like a great Strategy RPG
Pokémon X/Y - It's Pokémon in 3D
Etrian Odyssey IV - Never played the earlier titles, but a very interesting game to me
Mario & Luigi 4 - Mario & Luigi 3 is one of my favorite games
Pokémon Mystery Dungeon 3 - Mystery Dungeon 2 is one of my favorite games
Ace Attorney 5 - I love the Ace Attorney series

All of the games I mentioned has been released in the West and been announced to release in the West.
I also own Ghost Recon and Virtue's Last Reward for the 3DS, but the former is not a must buy and the latter in available for the Vita.

I'm not so much into platformers, but 3DS has Super Mario Land 3D and NSMB2, so you should be set.

Also not to forget, you can access the whole DS-library.

I just want to remind you that you can play almost every single DS game on the 3DS as well! If you can consider getting one of those you have literally thousands of games!!!

I don't own a 3DS myself but I can list the games I am interested in:
Zelda: OoT (if you never played this you simply have to get it, you're not a true gamer until you've tried it)
Star Fox 64 3D (stupid name, but looks like great fun!)
Kid Icarus UR
NSMB2
Mario 3D Land

Mario Kart 7

Castlevania: Mirror of Fate (or whatevz it's called)

Heroes of Ruin (Kinda like Diablo...)

Other games that might be of your liking:
Donkey Kong Country Returns (coming this year, and although it's a port the original game got great reviews!)
Resident Evil Revelations (it's getting ported to PS360U - but this is the original!)
Fire Emblem Awakening (Just look at dem reviews!)
Kingdom Hearts
Pokémon X/Y
Luigi's Mansion
Monster Hunter 3
Animal Crossing NL
Project X Zone

Old DS games that might be of interest:
Super Mario 64 DS (classic platformer)
NSMB
Chrono Trigger
Mario Kart DS
Lots of Pokémons
More Animal Crossing
A few more Zeldas
Final Fantasy IV

And then you got a shit load of eShop titles as well...

Outside the ones mentioned, I can reccomend

Ghost Recon: Shadow Wars- It was an excellent launch title and fairly similar to Fire Emblem Awakening in gameplay

Crashmo- an eShop title that has numerous tricky puzzles but has a solid feel to it

I guess the impressive graphics and Uncharted GA sold me the Vita. I've played a lot of good to very good games on it so far:

Rayman Origins, Persona 4 Golden, Gravity Rush, Uncharted GA, LittleBigPlanet or Tales from Space Mutant Blob Attacks, just to name a few.

Now that I already own one portable device I play regularly with, I need to be really sure it'll be worth it buying a second one.

Is this game anything like Valkyria Chronicles? Because both story and gameplay were pretty boring imo. Only played until chapter 5 or so.

3ds is a pretty good system, But prepare yourself for the e shop

it's terrible, and that's putting it nicely