My daughter and I sat down to play Wii Party U for the first time this weekend.
Now, in the past we've played various Mario Party games and enjoyed them.
For Wii Party U we just played the long, idiot, zig-zag bridge race. The individual mini-games were usually pretty good but a bit lacking compared to the Mario Party games.
The overall zig-zag bridge bit (and there are more "overall" games -this is just our first impressions from the first game we selected) was putting us to sleep. It was all very generic from the zig-zag board, to the tile pieces that made up the spaces, to the dice rolling, etc. It was like the most uncreative board game you could imagine. The music was generic Nintendo waiting-room music. It all just slowly lulled us to near sleep.
Nintendoland was quite a bit more fun. I knew of a few of the good games and show them to the girls (Donkey Kong something, Mario Chase, Luigi's Mansion (not the right title)) and we had a blast chasing each other or trying to find each other or, as a single person, doing the Donkey Kong rolling game.
Based on my experience I'd say, "Just get Nintendoland.."
The one caveat is that there are some games in Nintendoland that are kind of duds and we really didn't explore Wii Party U to it's fullest - it was more a "first impression" kind of thing. A game shouldn't lull you to sleep, though.