If a set of features, A, results in an AUC of 0.5, then an improved set of features, B, were used resulting in an AUC of 0.75, how do I express this improvement in words: 

1. 50% improvement (0.75-0.5)?
2. 25% improvement (0.75-0.5)/0.5?

it seems like simple statistics, but I am mixed up on this issue.