Top Gas was a brand of green gas, but both are really propane.
Can't help you with the differences between the M9s, however...I don't know what it is meself. The HFC Beretta is a nice choice too, if you haven't looked at it yet, and I'd reccomend it on the sole virtue of being fully automatic
EDIT: Learn yourself before speaking, much? That doesn't kick in until 31 Mar 08.