Definitely DK!

Donkey Kong. Haven't played it yet but it looks so good.

DKC: TF is hard as hell but fun and only has 2 players max.
NSMBU is fun and pretty easy and has 5 players max

New Super Mario Bros. U would be better for playtime with the family...
Tropical Freeze is better suited to hardcore platformer fans...

I strongly suggest dk, it's not nearly as hard as some people say. A few of the bonus stages are super tough but for the most part it's no harder that rayman legends. Also both are good for two player but I personally feel that nsmbu gets frustrating with three or four players. Another plus for dk is its at least 10 bucks cheaper.

Tropical Freeze is the better game here, but since you want to play multiplayer I'd say NSMBU is the better choice for you, Tropical Freeze is way harder than Mario.

Thanks guys leaning towards dk now :)

This pretty much convinced me cheers man

Why not have both?

I believe Donkey Kong Tropical Freeze is far more polished than New Super Mario Bros. U.
They are both excellent games in their own right.
Both of them improve upon their predecessors.

Donkey Kong has two players max co op. It has more than two characters to choose from however and they all control differently (abilities etc.).
Mario Bros. U has a maximum of five player co op with all the characters controlling the same.

Just because i have enough platformers for now so want one or the other for now. They look very similar so hard to choose between thats all