Both are fast & easy code editors.
Notepad++ is powerful, by supporting tabs, code folding, extended syntax schemes and easier ability to connect it to developing tools and compilers. It also can search/search&replace over all opened files which is a great time saver, and the regular-expression engine is the best.
Notepad2 is faster, and thats about it. it does not support any of the above advantages of Notepad++, it has regular expression engine, but it is not as powerful as Notepad++.
Notepad2 is my personal favorite, well just because of it, opening heavy files or search/search&replace takes absolutely zero processing time.