Also note the webplayer doesn't require any tricks with reprogramming/replacing the BIOS. It's good to go out of the box, and it's ridiculously easy to put a hard drive inside it.
That said, I have both. The IO (which is well, apart right now) is on a desk and one webplayer is in a laptop bag so I can take it places (which I do). Screens are similar, IO screen seems a little better. The big DOC in the WP is great, and my second WP doesn't have a hard drive yet, running the Win98Lite image on it.
Also, the wireless keyboard on the WP is much better than the IO keyboard, especially the roller mouse ball. It feels more like a laptop keyboard than a desktop keyboard, and has all the keys.
WP has mic/headphone jacks up front, but these can be added to the IO. Parallel port on the IO, none on the WP. Two USB on the WP, one on the IO (again, can be upgraded to 2).
IO is very upgradeable. WP not as much - can't replace CPU. You can add a floppy drive to the WP. You can add two serial ports to both as well.
They're both good machines, and for the price of either you're not going to find anything that matches them.