I think it was melannen who said an effective summary should be like a joke set up so the reader will click to get the punch line. That doesn't necessarily mean that it should be funny; but it should make the reader want to know how the set up is satisfied.

Personally, I almost exclusively care about prose style, so snippet summaries are the most likely to get me to click fic that's got an interesting/good voice and the least likely to get me to click fic I won't like. But my preferences are kind of off-center for fandom, I think - I rarely read based on pairing or plot. I tend to try to compromise and pick a snippet from the fic that gives the set up as well as my writing voice.