That's the main difference - gamepad (gpwiz) vs. keyboard (keywiz).
For what you are using it for, either would suffice. However, I would go with the Keywiz for two reasons: one being that some motherboards require a keyboard to be plugged in on boot. Some will stop at the BIOS screen and beep waiting for you to press F1 to continue. This is only an issue with some motherboards, so that might not be a problem for you. If it is, you will need a keyboard plugged in if you have the GPWiz.
However, the second reason is the most compelling; some games will not let you map different controls than their standard, and will only take keyboard strokes. (Daphne comes to mind, it only somewhat supports joysticks and allowing control re-mapping).
In my opinion, the KeyWiz is the way to go...