purportedly, the slab-sided 4.5" barrel mk3 is closest to a 1911. the 5.5" is a little muzzle-heavy. i prefer the muzzle-heavy feel, to be honest. i shoot the 5.5" very well, to me its the ideal length and weight of the 22/45. and i like the adjustable sights too
that being said ... aside from the controls being in the same place as the 1911 ... its really hard for me to compare the two guns. i'd buy the 22/45 because you like the feel of it and because its a real good plinker in its own right, not necessarily to imitate the 1911. something like the kimber pro target would probably much more closely imitate the feel of a 1911.
comparing mark II to Mark III: the mark II has a thicker grip than the mark III. the mark II grip also is very slippery if your hands start to sweat any. i had to remove the LCI on my MKIII to make it function. Mark II has a better trigger. Mark II is also a little easier to reassemble, although they can both be tricky until youve done it a few times. the mark III is more slender overall, and I prefer that. magazine disconnect and LCI are a PIA, though. so the factors go both ways.
i own two 22/45's (Mark II and Mark III) and two 1911's, so this is just my calculated opinion