If you are a microbiology student, you more than likely have a solid grasp of molecular biology and should have taken at least a basic biochem course that will satisfy what you need to know for the GRE.
I heard the Biology GRE is easier but I have a stronger background for the Biochem/Cell/Molecular bio one. So although the test is harder, the material won't be as alien to me. However, I wonder how much of the material I've retained--especially the biochem.
And although I've touched on less of the Bio test, it seems as though it will be more interesting to study for.
I'm not 100% certain but I'm leaning towards the biochem/cell/mol bio subject gre.