Class one has tighter hot pixel and column defect specifications. Generally speaking I don't recommend spending the extra money on class one. Cooling reduces the hot pixels and bad columns, and then calibration (dark subtraction) finishes the job and you can't tell. Also over the years your sensor will gradually accumulate new defects caused by ionizing radiation (cosmic rays, but also radiation from building materials etc.) and will eventually end up being class two anyway. I have a 17-year-old KAF-6303 sensor (2001 vintage) that was nearly perfect when new - zero hot pixels - but now would only qualify as engineering grade as it has many bad columns. (And it still calibrates out perfectly.)
