It all depends on what you want to measure. AIS is a severity scale, and the most commonly used. It is at anatomical severity scale. ISS is a combined outcome scale. If you want to measure outcome based one AIS scores (like ISS) I would rather recommend NISS or the newer severity measure, the Trauma Mortality Prediction
Model (TMPM), uses either the Abbreviated Injury Scale (AIS) or DRG International Classification of DiseasesV9th Rev. (ICD-9) lexicons.