It looks like the hComment microformat wiki entry hasn't been updated in a while. It also looks like hComment and hReview are very similar.
There seems to be more interest and activity with hReview. However, the data that I'm representing is clearly a comment, not a review.
Should I use hComment (which appears to be a better semantic fit) or hReview (which appears to have more interest and activity)?
I suppose the "right" answer is that you should take it upon yourself to reinvigorate the hComment community. Of course, we all have full-time jobs, so that's a bit hard to justify to the bosses.
My take is that, since microformats are largely philosophical exercises in semantics and not particularly (at this point) relevant to extant tools, engines, or databases, you should go with what's accurate and closest to reality rather than compromise in order to have better discussions and interest.
Considering that Google has started supporting miroformats, use the type that best fits semantically.