Both are good guns...
Both are good guns when they are working. I have shot both multiple times before purchasing a Mark III 22/45 5.5" bull barrel. Disassembly on both is a real pain. Accuracy on both is quite good. Where the Mark III really shines is in the design. The buckmark has screws that hold it together which come loose, and has a spring that is held in place by the grips. I have seen both of these to be major sore spots. The rifle + pistol club at my school owns several of each model, and the rugers have significantly fewer mechanical problems. My 22/45 took a good 800 to 1000 rounds to get worn in properly and start behaving, but since then, it has been a ton of fun, and significantly more accurate than I am. I have probably shot 5000 rounds through it so far, and had no problems.