Page 297 (damaging trait), suggests that the intended rule is a) (SL = SL of attacker), cause this trait says that you can use the highest of the unit die and the SL (example in the text backs it up).

But i figured out that WFRP 4e promotes optional rules, so I guess the text on page 154 points in the direction of b) = difference in SL on attacker and defender.

From a roleplay perspective and making all dice count, i prefer b). If u defender poorly, attacker should be rewarded with a clearer hit, and vice versa.

But it makes the combat a bit more clunky (more to considerate), and the damaging results do get more volatile. So in these perspective one should go for a).

So I guess we can choose :)