Well, some of this is personal taste as both units offer many of the same features. I have played with the MPx300 several times earlier this year. It wasn't stable at that point so it did crash a lot. I'm sure the finished version will be stable though. The things I didn't like were the scant 32 megs of RAM (even for a non-power user that's hardly enough space these days) and the thickness of the unit- I like clamshells but it's a chunky monkey. Also the display is quite a bit smaller at 2.8", so you'll need to have good eyes.
Both have similar processors and clock speeds, BT, WiFi and a camera. And each has a keyboard with the iPAQ going for the more traditional (and removeable) snap on thumb keyboard which the Moto has that interesting integrated keyboard that was OK to use. The Moto is geared towards those who are more phone-centric in their needs while the iPAQ is targeted more at PDA users given their industrial designs.