I think so. You'll always have people who will say 'boring 2d mario, done that before' but that's the point, its still fun. The coinage thing with NSMB2 is interesting to say the least, they only barely showed what is really in the game so that would mean there is still a lot to surprise with.
At the end of the day, if you like the NSMB series, then both games will be worth it because each one will have a full game's worth of levels for you. Personally, I'm not psyched with the screens & videos I've seen of NSMB U. Nothing looks that great given that it's Nintendo's first "HD" game...Just my two cents.
NSMB2 looks fun though...has me thinking about buying another 3DS! (Sold mine a few months ago)
You shoud wait till August 19, 2012 for the Nintendo 3ds XL and you can bundle it with New Super Mario Bros. 2. Details about it are in the Nintendo Direct in this link: http://www.nintendo.com/nintendo-direct/archive/06-21-2012/ Hope this helps!
I personally love the concepts of both games. The coin collecting emphasis for NSMB2 is something I know I will enjoy. I only played about half of NSMB Wii co-op with my brother, so the look of NSMB U is not stale for me. Plus, the option to use the GamePad seems like a nice way to get my parents or my girlfriend playing a traditional Mario platforming game. I pre-ordered NSMB2, but waiting for the official price of the Wii U system and games before pre-ordering NSMB U.