2nd# came with extend, along with the dlc for 1st, so they're kinda like one package! Its dlc aside from 1st in general is the best, it has the best mixture of the dlc songs available in all 3 games. (Time limit yass).
It also has a good selection from all 3 games, but with updated maps and models so they look like extend. It's really pretty!! It takes all the save data from all 3 too!! It's so handy to go from 2nd to extend to 2nd# and keep my high scores :D I don't have to switch discs as often with 2nd# around!!
The dlcs it came with for 1st are great, too! The hello world platformer was 10/10, and the toeto tamagotchi thing was a bit too hard for me to read bc of the font but it was still gorgeous. I use the luka psp theme xD The edit datas those have for 1st are really good, and there are so goddamn many! A whole set for miku, and a mixed set for luka and rin and len (including songs from 2nd and extend!) The MP3 are also on your psp then, so they're kinda like buying 2 compilation albums.
2nd# was really a nice bow on top of the original trilogy that tied them together for people who bought extend!