Well, it depends on what you want it for. The PSP Go has bluetooth and is lighter but the PSP 3000 can be hacked and play homebrew games and ISO'S AND CSO'S.
PSP 3000:
PROS- Can be hacked, Plays homebrew, built in mic, bigger screen, can play actual UMD games and UMD games that have been ripped (ISO's and CSO's) from the memory card. Cheaper $169.99
CONS- Has a screen problem that makes scan lines, can't install M33 custom firmware only GEN, A little bigger than the Go.
PSP Go
PROS- It has built in bluetooth, faster internet, No UMD ( For UMD haters). All digital media. Comes with 16 GB of flash memory
CONS- Can't play any UMD's you may already own, most likely cant be hacked. Uses M2 (Memory Stick Micro) which can be easily lost and if you already own Memory Stcik Duo, you cant use it. More expensive, $249.99. Smaller screen.
Overall, each system is good in it's own way but if you are planning to hack you PSP and like bigger screens and like the huge UMD lobrary available and only got like $200, get a PSP 3000. If you like all digital media, BlueTooth, no UMD's, and a smaller screen and don't care about the price, wait for the PSP Go to come out, Comes out October 2009.
Hope this helped!