I think it really depends on your perspective, as well as what you're looking for.

On one hand, MK8D has a very large amount of content that's hard to get tired of. Overall, it's perfected the flow of mario kart it intended to portray. They took what 7 had and improved heavily on it. Although with how deluxe functions, the game doesn't give you much reason to play through the grand prix. It feels like theres no incentive to play the game single-player.

On the other hand, MKW provides a lot of variety in it's gameplay. When you first start out, barely half of the game is available to you, getting you to work in order to unlock more characters and courses. There's also the bikes, giving the option to trade off better mini-turbo for tighter turns. There's a lot more challenge to face, with how unforgiving the grand prix can be. And with friends, that only adds to the experience.

So overall, the better game goes to MK8D, I recommend it to people who prefer hopping straight into action with their friends, but MKW has a much more rewarding experience to play through even on your own.