Sounds like a silly question at first considering how both games have been critically received, but here's what I have to consider:
-Pricing. I can get FFXIII for $10 while Ni No Kuni is $60. My spending money budget for the next two weeks or so is $150. -Casual play. I used to be a hardcore gamer in the RPG department, but haven't seriously sat down and played in a while due to school & work. For the same reason, I'm not sure how much time I can actually dedicate to playing the game I choose. -Familiarity. I've played and beaten FF 1, 2, 3, 5, 6, 9, and 10. I've also gotten into 4, 7, and 12. Basically, I'm very familiar with the FF series and enjoy it a lot. --- Who got the 500th post on final Strikeforce event topic EVER? <---- This guy.
FFXIII will probably get a lot of crap in this thread but I rather liked it. It was no FF6 but as long as you know what you're getting so that you aren't mega disappointed by the linear nature of it it's a pretty good game. Lightning is boss.
I recommend doing some research on the games and make your own decision. Aside from a few trolls, you will get a very heavy lopsided "Ni No Kuni" response.
I personally like NNK better than I did FF13. This FF was the first one that I beat the final boss and had next to no desire to play anymore. I was not immersed in the story. Lightning was the only character I gave a crap about and it all felt incredibly linear.
I knew I would love NNK because I enjoy Level 5's RPG's and I absolutely love Studio Ghibli. So it was a slam dunk for me.
However, these games are not at all alike, so it really boils down to your preferences right now.
This game. FFXIII is not a bad game, but it's way too linear and the battle system, while fun, doesn't require much thinking. The one thing XIII has over this game is story, characters, and voice acting. --- This is your life, and it's ending one minute at a time -Tyler Durden
Should get both. Both are great games! Especially when you can get FFXIII for $10. Don't let haters sway your decision. But do not expect too much from both games because personally if I play a game with low expectations, I usually end up enjoying it even more.