I got to play the PSP Go at a Playstation Blog meetup last friday during PAX and here's my observations:

Weight difference really isn't an issue. While the Go feels ridiculously light and amazing in your hands, the 3000 has a nice weight to it as well. Either one really wins.

The deal-breaker for me, though, is the controls. After playing both for longer periods, I absolutely have to say it feels better on the thumbs playing games on the Go, instead of the 3000. On normal PSP's, it feels like I'm bending my thumb too much inward at the edges to get the controls, and it hurts after long play sessions. On the Go, however, because the buttons, and especially the stick, are more in the middle, your hands are more over the controls with your thumb coming down, more extended.

Maybe I just hold a regular PSP wrong, but when it comes to controls, the Go is 1000x better to me.