DJMAX Portable 2 brings a fresh set of songs with tons of content.
As far as songs go I personally like the songs in the 1st DJMAX. I havent gotten all the songs yet so i may be speaking too soon. Theres also alot of extra stuff in the game.
DJMAX Portable 2 has 4 buttons all the way up to 8 buttons for every song.
There are a ton of unlockables including pictures which are really cool, which can also be saved to your PSP photos so you can make them wallpapers. Theres also an OST mode that features all the songs in the game and if you own the 1st DJMAX Portable, you can transfer songs in the 1st game into the OST of the 2nd or something close to that. Theres also a link battle for 2 people. If you want to just see the animations for the songs you can go to MV Mode where you can watch and listen to songs without having to actually play it.
OVERALL: If you've never played DJMAX before you'll love this. I'd even reccomend buying the 1st one too. If you have played DJMAX before I'd still reccomend DJMAX Portable 2 because of the new songs to play + more button modes and all the unlockable content.