Most important thing is to hit your opponent. It really not metters did you miss with .45 or .32; next, hit placement is also the major factor - again, .32 in the head is far more productive than .45 in hip. In short-range combat you need to incapacitate the opponent in shortest time possible - again, two hits with .32 works better than one with .45. Today lot of soldiers carry protective vests, superb helmets, etc. - your pistol slug could qualify just as "pain in the chest", regardless of caliber.
Why mentioning .32 ACP caliber? Well, majority of used sidearms in WW-I (real close-combat) were .32s. Situation in WWII was improved only a little - again, many handguns carried were .32s, .380s, 8mm Nambus, etc. More people is killed with a .32 than a .45 (regretably, lot of cases were shoting up the captives). Even the soldiers armed with 9mm or .45 pistols received so little training that they probably could not hit a moving target beyond "point-blank" range. (Lot of policemen today are no better with their handguns; anything serious than domestic dispute ends with "calling the backup"; if the firefight did occur, it either one-shot kill at two feet or 100% misses at 5-10m).
So, if we talk about "Combat handgun", we're not talking about either Military or Police, maybe just the specialised formations inside them - who used them rarely. "Combat handgun" is a category mostly for civilians, who are not allowed to carry something really efficient - and have to carry them hidden. (Only real advantage handguns have over the rest of small arms is concealed carry.)
So, I'm a civilaian and I need a "combat handgun" (What for? Let's say my ex-wife is a maniac and have access to various sizes of kitchen knifes). What to choose?
1) I want to carry it hidden - so, nothing too cumbersome - it has to be compact and light.
2) I don't have time & money for serious pistol training - so, nothing too complicated or demanding.
3) I can spot my ex-wife on qiute a long range (and run away from her) - to caught me, she would be using both short distance and surprise - so I need something easy & fast to operate (nothing "cocked & locked", "conditon three", etc.)
4) She is not a expert fighter too - realy good to cut something with the knife (or stab it in my back as a "target of opportunity"), but she could also "borrow" a handgun from his new victim (boyfriend) at best - so, as I should not expect an attack with assault gun, LMG or RPG, I really don't need something with great determent/firepower/range/price tag.
Most advisers would suggest .38 Spl snub revolver, .380 DA-only pocket pistol or even some of now-available 9mm compacts. I think they all are fine, but my ex-wife could be a tough bitch, so I would probably go for .44 Spl "Bulldog" revolver (5-shot, 3-inch barrel) firing 250-grain LSWC slugs (standard pressure, not +P rated). I think I could menage this(some vizits to firing range would do the trick - plus it's fun too), and - in case of passionate women - size do metters!
(plus rather cheap ammo). 
Boy, these Zastava guys would sell anything to make a buck! (One of their dealers - my friend and above mentioned rifle owner - got from them a very long barrel for CZ-99 pistol. It protracts 4 inches from the frame and outside part has smaller diameter then the inside barrel. He said it is designed for better accuracy 