I'm getting the DS one. It's better IMO.
Then again, I only own a DS Lite :p

i wish they'd put the hats in the ds one though, it almost makes the psp one worth getting but i gotta go ds more customization

The PSP version has more things that can be customized/selected for your teams and schemes. Some may prefer stylus for drawing landscapes but analog nub/d-pad is easy enough to draw with. You can save the PSP landscapes to memory card, so you should be able to copy them to other PSPs and eventually edit the landscapes on a PC.

The worms seem to walk slightly faster on the DS version (maybe imagination). I love the minimap on DS.

Online play seems to start quicker on the DS version and less delay between turns. The PSP version has clan support, host migration, some message support (you have to create the messages before you connect to the network) and more online options.

There are many pros and cons of both versions but I'm too busy playing to list them all now. They are both great games.