Honestly, while I have hacked versions of both platforms and I enjoy them both immensely, the PSP edges out the DS for me because of the sheer number of crazy things you can do with it. At various points in the lifetime of my PSP, I have:
- made it into a webserver with Apache and run webpages over it
- used it as a torrent repository for music (16GB memory cards FTW) - linux OS, Azareus torrent client (now Vuze)
- used it as a wireless repeater to boost the range of my wireless network so that I could use my laptop on the far edges of my property
- had windows XP installed on it so that I could work on a spreadsheet on a plane ride
- (currently) used it as a MAME emulator to play classic games
I am on battery pack #3 with this thing, but it's cool as f*ck.
The hacking for the DS is mostly to do with getting free games or stuff form Japan that won't normally run (i.e. the DS web browser, which I think the DSi just has by default but I haven't been paying much attention). And it requires specialized cartridges, where the PSP can be 100% software hacked. The only thing that I have at home that comes close to the hackability potential of the PSP is my Wii, as it too is fully software mod-able.
Yes I have a Wii, and no, I am not ashamed! Especially now that I can play
this game.