I believe the mojora's mask one is sold out. I checked 1 hour after the announcement and couldn't get one sadly.
reason to upgrade.
it has exclusive games (so far just xenoblade though) also the added functions (amiibo, c stick etc.) work in normal 3ds games. the c-stick and shoulder buttons act as the cpp so resident evil, metal gear, monster hunter and others can use them as if they were a circle pad pro.
I don't know if this is confirmed officially but certain games will use the added power to run smoother. apparently mh4u runs at a constant 60fps on the new3ds but not the original, other games such as pokemon do not sadly.
basically do you want to use gateway or not? currently there is not Huge reason to upgrade on a general level. I'm doing it's a great deal, $100 after trading in a 3ds xl. but my xl is up to date anyway.