Both are amazing.
I had a blast with SM3DW playing with my girlfriend, my sister, my brother and even his girlfriend.
On the other hand i can just turn Wii U on and blue shell some over the internet after a long work day, and this is great.
Going to pick SM3DW because it held me until i got 100% and even after, despite the fact i am playing way more MK8 atm. Both are a must for Wii U owners, along W101 and WW:HD.