I got both. Each version is just brilliant and excellent. The PSP version is just absolutely beautiful and the music and sound samples are much better than the DS version. As expected, the resolution, screen size, and zoom capability makes the PSP version much more like playing on the PC or the 360, and it makes the game easier. I'll be playing both with friends though, and recommend each release on its own merits.
If anybody is wondering, this is the same game, not in name only. Everything I've seen so far of the campaign is the same, and both have full online multiplayer and level editing features.
It is very cool to see a real campaign mode as opposed to the 'challenges' of recent releases, and ditto for the 'puzzle' mode.
Minor nitpicks on the DS version:
1) The control is different from both PSP games and the 360 XBLA quasi-port of Open Warfare 1, but it doesn't have to be. The four main buttons could work exactly the same on all games, and this would make it easier for people who play multiple versions.
2) The physics seem a bit different. The PSP version seems consistent with the other recent Worms games, there's definitely a difference here but only the people playing multiple neo-Worms games will notice.
3) The blowtorch wants to lock into a couple of different angles instead of allowing free control. The PSP version and other Worms games don't do this.
Regardless, highly recommended for either platform.