What is the possible range of grades? "5.0" is typically the highest possible grade but it would seem strange if the teacher gave that as the "average" grade (can you say "grade inflation?). If that were in fact the highest grade, I would choose the one with the smaller standard deviation since that would give a smaller change of being far from that average. If, in fact, 5.0 is just a "possible" grade with both lower and higher grades, the standard deviation won't make that much difference.