I've owned both, and here is my assessment.
The Sansamp is strongest suited as a tool to make a non-stellar amp sound decent. It gives you that DI out that some amps lack, and acts as a preamp if needed in a pinch. It can give a good Ampeg-like grit (but you have a GK....)
The M-80 is a different beast. It gives great distortion (much different than the overdrive of the Sanamp). For heavy stuff, it is great- love running FF through it. Plus, the EQ was pretty good, and can help you cut cleanly through a heavy band setting when competing against the Marshalls and Mesa my two guitarists used. It is a strong stompbox in and of itself.
So depending on the music you play, judging from your rig the M-80 would be a better choice.