I have both, and each has advantages and disadvantages.
The MK II has the mag release at the base of the grip, like the P-38s and other European guns. This is a PITA.
The mags from the MKIII seem to fit the MK II, but the MK II mags will not latch into the MK III. An "Ulitmate Cliploader" that sells for about $30 is an excellent accessory for fast reloads and works for both mags. It won't work on a MK I.
The MK III has the release behind the trigger and is great, but it has two "improvements" that I don't like. The internal lock and the loaded chamber indicator are unnecessary additions which add to the manufacturing cost and provide no real utility.
My MK II bull barrel easily accepted a fiber-optic front sight, and I think only the pencil barreled guns of all marks will not accept the FO.
The 1985 MK II seems to be better finished and never gives a problem with reassembly (once I had it figured out and practiced a while), but the MK III
seems just a hair sloppy and you really have to be particular about aligning the barrel to the frame, or the damn pin will not go in all the way.
In conclusion, both models vary slightly and I won't part with either.
Geoff