I've played the Wii U version, and I love it. Admittedly I did most of it in 2-player co-op, but it's still a terrific platformer.
~Gameplay: Poochy aside because I played the Wii U version, it's pretty standard Yoshi's Island fare. Jump, flutter, eat enemies, make eggs, throw eggs, etc etc. Controls are responsive and feel good.
~Graphics: I dunno how this would look on 3DS/New 3DS, but it looks terrific on Wii U. The worlds are as bright, colorful, vivid, and realistic-looking as advertised, and the frequent unraveling of yarn are fun to watch.
~Soundtrack: I haven't played Kirby's Epic Yarn so I can't really compare the visuals (I would imagine them to be similar, as they were both developed by Good-Feel) but I know both soundtracks and love them both. Tomoya Tomita composes a lot of bright and cheerful tracks, and the best word I can use to describe them is mellow. Sometimes a bit too mellow. But it's definitely among one of my favorite soundtracks, I love that kind of stuff.
~Difficulty (and stuff): Not that hard, but not stupidly easy (iirc). Collecting all the items is a fun challenge. I haven't collected everything yet, though I haven't played it too much. There's a sizeable amount of levels and things to find, enough to keep me busy for a while.
So yeah. I'm probably not buying it because I already have it on the Wii U but would absolutely recommend getting it.