Just to clarify (been on the DPA website):
DPA4090 - NOISE 23 dB(A), SENS 20 mV/Pa, MAX SPL 134 dB
DPA4091 - NOISE 26 dB(A), SENS 6 mV/Pa, MAX SPL 144 dB
So the 4090 is less noisy AND more sensitive while the only advantage of the 4091 appears to be its max spl handling.