Sonically, the main difference between the KM 84 and KM 140 is that the KM 140 has slightly greater response around 9 - 10 kHz; the difference is a tiny bit more than 1 dB. Of the two, the KM 84 is closer to being flat or neutral sounding although it, too, has a small bump in that region (not shown in 1970s or later response graphs; it falls entirely within the 2.5 dB tolerance field but it is definitely there).
The KM 84's capsule did not have a metal membrane; even the predecessor models KTM, KM 64, U 64 and KM 74 had Mylar membranes. The only Neumann small cardioid with a metal membrane was the KM 54 microphone from the vacuum tube era.
Electronically the KM 140 is more sensitive, quieter, has a considerably higher SPL limit, and is less readily affected by cable capacitance, but it requires more current from the phantom power supply that drives it. The acoustically identical, non-modular model KM 184 is 3 dB quieter even than the KM 140 and otherwise has the same specs and sound. The KM 84 had a transformer output while the KM 140 and KM 184 are transformerless.
Personally I prefer the KM 84's high frequency response characteristic, and wish that Neumann had never changed this, though I don't doubt that it was requested of them. Apparently at one point an important Japanese customer (radio station or network) ordered a batch of microphones using the transformerless KM 184 electronics but the replaceable capsules and rear entry port arrangement of the original KM 84; I wish that combination was available to everyone. (It isn't--I've asked.)
--best regards