Occupied elements of the MU (that is, values with something other than a space value) will occupy the same space whether a MU with ordinary or NU compression or if stored as a single field in a PE. So the difference is how much room the “empty” occurrences will take up.
Empty fields with normal compression (not NU) will occupy 2 bytes (length + value space) per occurrence. If you are storing 100 occurrences, it will require at least 200 bytes per record (plus the occurrence count of course). With NU, there will just be the occurrence count (0).
A PE with a single NU field will have an Empty Field Count for each group of empty occurrences. Best case, 1 EFC for 100 occurences, worst case 50 EFCs with alternating empty/filled occurrences.In an NNU MU with similar alternating empty/filled occurrences, you use 100 bytes for the empty fields (50 x 2 bytes). With PE/NU MU, you use 50 bytes for the EFCs.
Does the 50 bytes per record matter? DE/SP on a PE field can’t be used for READ LOGICAL; DE/SP on MU can be. Otherwise, I don’t think there’s much difference.