Not sure why USS Enterprise cites "MAME default on the (p)rocessor" as an advantage over the IPac, but to each his own.
I can guess what is going on here - First off, the I-PAC/2 and KeyWiz both support the MAME default set out of the box. (Neither supports P2 buttons 5-8 b/c MAME doesn't define defaults for these, so you have to manually set them, but that is a MAME issue, not an encoder issue.
The I-PAC/4 has a lot fewer inputs defined, mainly b/c MAME failed to define these.
The KeyWiz does define a lot of useful keys such as P (Pause), Esc, Tab, and Enter, hard-coded, but it has four extra inputs to play with. OTOH, the I-PAC can be re-programmed as desired and will retain it's settings, and the KeyWiz will need to be re-programmed from a startup file if you don't use the default codeset.
BTW, short and sweet comparison is here:
http://www.mameworld.net/tigerheli/encoder/main.htm#KeyWiz_vs._I-PAC2 but you will likely need to read a lot of the rest of the article for definition of terms.