I had a 1000 series and upgraded to a 2000 series and find its the best of the 3 regular PSPs released. Its thinner than the 1000 and has a better display than the 3000. Read this review for what I mean on the screen:

Linky

I also have a DSi but in comparison I get more enjoyable game time with the PSP. I find the games on DS too gimicky whereas the games on PSP tend to be more geared for the extreme gamer. I guess it depends on what you like to play. My games of choice are less cutesy and gimicky like those of the DS and Wii and the multimedia functions of the PSP appeal more to the tech side of me. I keep the PSP with me in the car whenever I go out to pickup the missus and play it in the can as well! The addition of WiFi is an added bonus too and I can even listen to music and watch movies in widescreen glory in the bedroom if I feel like it. But thats just me, make your judgement on what type of games you like.

PS. All of the PSPs can be hacked, no model is more hackable than the other. And as for getting a japanese one, if you are used to using the X button as your confirm button, then I'd advised against getting a jap unit. I have a jap unit and whilst you can upgrade the firmware, the confirm button is the O button which can get frustrating when you are used to using the X button (like on the PS2 and PS3). Not a deal breaker but something to consider (and no it cannot be changed unless you hack the firmware).