I've owned all three (four if you count the 22/45 as different) and I can't really tell a dimes worth of difference in them. I'd look for the one I could find the best price on, but I wouldn't cross the aisle at a gun show to buy one over the other otherwise.
I ALWAYS look used first. I picked up a 1960 standard auto (AKA MK 1) the day after Christmas, with the box and manual for less than $200.00 out the door.
My wife has a MK II that she's put thousands of rounds through. Any problems were fixed with a drop or two of oil.
I don't have pictures of them anymore, but I have owned both a MK III standard, and a MK III, 22/45. Same as my wife's, the only trouble I ever had with them was when they got REALLY dirty. A drop or two of oil fixed them short run. A good flushing with Gunscrubber (poly safe) took care of it long term.
You will hear people say they are hard to dis/reassemble. They're not hard. They do require you to follow the directions, TO THE LETTER. Especially important with the MK III. Like reading the Bible, every word is important.
Or do like I do. Just flush them out with Gunscrubber (the poly safe version is safe for all) wipe them off, put a drop of oil here and there, and leave it alone otherwise.