I have both cassettes. I find the 12/25 is adequate for riding most hills with the exception of some local monsters. Of course, I don't have a damaged knee. For $40, I'd go with the 12/27 and try it out. Is your crankset a 53/39? It kind of depends on what you have for a small chainring. If its bigger than a 39, you might want to consider just changing to a compact or a triple crankset.