mW is power input. V is voltage input. Power depends on two of the three variables(V,I,R), and then there is only voltage dependency for sensitivity. One maybe more efficient than the other, but not necessarily more sensitive while at it.
Ok this is becoming a sound science topic but is a very basic one. V, I, R are all involve but you only need to get 2 values of any combination of the 3 to compute for power. These are not absolute values because there are other factors involve (e.g. cuurent limiter circuit etc) but the headphone sensitivity/efficient spec are nominal values and they are good enough as a general guide.
To make things simple:
96dbSPL/mW headphone definitely will play louder than a 97 dbSPL/V all things equal. If you don't agree the best thing to prove it is get the 2 exact headphones in question K545 and DT880 and see which one sounds louder.