I have both and IMO 3d World is far superior to NSMB U, but if you really like classic 2d mario gameplay then it may be worth it.
Before playing 3d world I was disappointed that it wasn't more 'original' like mario galaxy, but after playing I love how polished they have made the experience, and it's a lot of fun multi-player.
The Wiiu certainly isn't doing too well, but for me at least Mario 3d World shows why Nintendo is still a relevant force in gaming.
The WiiU outsold Xbox One during Christmas you call that not doing well check QJ.net
Edit: If anyone in Aus is looking for the game at a decent price, it's just over $53 shipped on the video ezy website. They are generally cheaper than ozgameshop etc. I ordered it from there just before launch, did take about 2 weeks to arrive (shipped from Hong Kong) but best price I could find.