"Mario games" is too broad of a statement. Some Mario games are great (Galaxy 1+2), and some are just generic fluff (NSMBU, NSMB2, Mario Tennis 3D). The nicest thing I can say about NSMBU is that it's a very competent 2D Mario game. It's not doing anything new, and it doesn't even show off the pad very much. The problem isn't the content, it's the price of the content. Should be a 40 dollar title. If you buy it, you most likely wont be disappointed in the game, just the price.
As for Darksiders 2...don't pay full price. I'm a big fan of the first game, and the 2nd was a letdown. It takes place in a huge, empty world with very little to do outside of the dungeons. If you can get it for 20-30 dollars, jump all over it. If the choice you're making is Mario vs Darksiders 2 and you're dead set on getting one of them, Mario is the answer.