Most likely it is your scanner on the 120 issue with how it is mounted. Your film really has to be dead flat to scan well, and 120 is a larger surface area and therefore has a better chance of not being totally planar during the scan. I warm up my negatives and then stick them under a book or other totally flat surface with weight for a whole day.
Color films are however actually less sharp than their black and white counterparts. The line pair resolution of color films is much less, likely due to their being made up of coupled dyes and the orange mask that is placed over negative films.
Color negative and positive films are pretty similar in their characteristics of resolution, but it really depends on which film you are talking about.
Remember, "sharpness" is meant to be the perceived contrast of edges of fine details throughout the image. Monochromatic silver grains can render higher resolutions than multicolored dye any day.