Son,
I am glad that you initiated this thread. Just like you (and some others), I keep varying between .357 magnum (Smith 627 "N" frame and Ruger GP100) revolvers and .45 ACP (Colt and Kimber 1911A1s, Sig P220 and H&K USP Tactical) semiautomatics. It is my strong belief that all of these are truly outstanding defensive weapons -- accurate shot placement being the crucial variable.
My personal rating follows:
Concealment: Edge to the autoloaders, although both are possible
Accuracy: Edge to the revolvers, although both are more than adequate
Capacity: Edge to the semiautomatics, although six rounds should be more than sufficient
Reliability: Edge to the revolvers, although the semiautomatics are also excellent
Penetration: Edge to the revolvers (.357 velocity is superior)
Expansion: Edge to the autoloaders (starting at .451 is a major plus)
Proven Effectiveness: Tie, both rounds have demonstrated exceptional defensive prowess
Please note that I've consistently and intentionally used the term "edge", because the performance margin in all categories is small. In addition, please understand that I am evaluating this as a non-LEO; accordingly, the chance of requiring over six rounds is slight.
My conclusion is carry the .357 magnum revolver or the .45 ACP semiautomatic with which you feel the most comfortable. Make certain the loads you use provide excellent accuracy, expansion, and penetration and that they always work in your firearm.
I will be eager to see if one of our TFL experts can solve this quandary for me. I trust both the 357 and the 45 -- in aggregate, they seem to be virtually tied for overall effectiveness -- but I'm no authority.