Gah, I meant Battle Chronicle. It basically has all the previous stages and mecha from the previous two games, plus new stuff from ZZ, Char's Counterattack, and Gundam Sentinel. Get the Seed Vs. fighting game too. It is awesome.
SD Gundam G Generation Portable is just like the PS2 game except the stages are 2D instead of 3D. I believe they cut out some of the sidestory stuff and add some Seed stuff instead to make up for it.
Do you like strat. RPGs at all? You should get Disgaea Portable, FFT, Wild Arms XF, Jeanne D'Arc (don't listen to the haters, they played the game for like an hour and went "turn limits? fuck that" and never touched it again), and Final Fantasy Tactics (which has not aged nearly as badly as the elitist Evilbore squad would have you believe).
Tales of the World: Radiant Mythology is also really great. Get Dracula X Chronicles too, Me & My Katamari, Mega Man Powered Up, Street Fighter Alpha 3 MAX, and the Sega Genesis Collection.