NSLU levels seem pretty good so far. Just the right amount of challenge and I like the shorter length. The smaller levels I think forced tighter level design.
Physical copy isn't available until August 25th. I would say the DLC is worth $20 if you like to get all the star coins. As a stand-alone disc for $10 more, I guess it comes down to if you are a collector or want the ability to give it to someone else. I recommend you get NSMBU first since it has a lot of extra stuff like the challenge levels.