There's precious few things you can get on a JP 3DS that you can't get on a US one if you're willing to wait 6-12 months, but as an owner of both, the JP unit has a few neat things over its US counterpart.
-I'm not going to mention how great having both Taiko 3DS1 and 2 are ^^ They'll probably be the most worth you'll get out of the 3DS.
-If you like dressing up your 3DS Home Menu, get some eShop Credit and download Badge Center. This one allows you to put decorative badges on your Home Menu, while serving no functional purpose, they are adorable. You get lots of stuff, from Mario, to Zelda, Animal Crossing, Tomodachi Life, even Splatoon.
-JPN Home Menu theme shop is also a whole lot more fleshed out than the US one.
-If you can read JP, you will also be able to play many games long in advance from the US versions. Examples (citing currently unreleased games in the US): Yo-kai Watch, Pokemon Super Mystery Dungeon, Bravely Second, Monster Hunter X, Rhythm Heaven 3DS and many more
-Also who knows, you might find the odd JPN-exclusive game you'll like (Puyopuyo Tetris being my personal recommendation)