Thanks everybody for the info and advice.
Magnum got back to me. The -U means it ships in a UPS box, and the -G means it has a GFCI on the unit, which would actually be pretty cool (for me, at least, because during winter I need to stick a 100w bulb inside my Girard tankless heater to keep it from freezing, but probably not a big deal in general.)
Hopefully Google will find this page and folks will have an easier answer for the difference between the "MS2000-G" and "MS2000-U" (or the MS2012-G and MS2012-U).
@airdaile, it's about 7 years old, unfortunately. So no warranty for me.
