There are no redone levels or anything, both games are different in a lot of ways. The complaints that you heard were probably people referring to the fact that the NSMB games have similar art styles and music. NSMB2 and NSMBU both have completely different level design though, so don't worry about that.
If you want more 2D Mario, go right ahead and get NSMB2. Many people would argue that NSMBU is better, but with NSMB2 you're still getting over 80 new levels, different power-ups, Coin Rush mode, and you can expand your experience even further with the already released DLC.
With this game, I always recommend renting it. The main game is short enough to beat in a day or two, then you can decide if it's worth the investment to pay for the full game and 100% it. It is fun, it's just nothing spectacular.
Tissues to the extreme! Super Smash Siblings is the s'iest game.