Whats are the differences between Monster Hunter 3 Ultimate for the Wii U and 3DS?

The Wii U version is the same thing with better graphics. It's just an HD version of the 3DS one.

It's not turn based. It's a real time action game where timing your attacks is everything, since some of the larger weapons are very slow, and monsters aren't going to wait for you to finish swinging before they claw your face.

Exploration isn't really a thing. The game works by giving you a quest in town, either to gather some item or kill a specific monster or something. Then you go outside town to the specific area where your quarry is and track it down. The world isn't really big though and every area is divided into smaller numbered zones. The challenge lies in finding the different monsters and defeating them, since different monsters are more vulnerable to certain weapons, traps, etc.

So basically it's an action game with very minor RPG elements in the form of making and upgrading your equipment by killing monsters and gathering their parts. There's also co-op but only local via wireless. Online play is only supported by the Wii U version, although it is possible to play the 3DS version online through the Wii U with a special app that's available for free.