I will go as far as to say the PSP Go d-pad is factually worse. It's not bad on its own, not as good as 3000, but not bad. the position ruins it because many PSP games have controls build around the layout where the nub is right under the d-pad. For example: I was playing Pursuit Force 2 (sometimes great game, othertimes very annoying) and that game has "look back/rearview" mapped to down on the d-pad, right above the nub. Metal Gear Peace Walker has down mapped to crouch. Since it's so close to the nub that it could be accessed without even moving your thumb from the d-pad, many games use it as an action button (while left/right/up are options or equipment). I'm making it seem worse than it actually is, but my point still remains.
and yeah, I said earlier that the nub felt better and I stick with that. I'm hoping the 4000 has an improved nub, because as much as the 2000/3000 are better everywhere else, the analog nubs on both of them aren't as good as the PSP 1000 or Go.