I know this is an older thread, but having just gone through this task this is what I found.
First, decide what you want to use the pistol for - carry, competition (if so which one), home defense, etc.
My primary use is competition...local and fun. Get me out of the house and around like minded folks, it's something the family can do (i have an 11 year old son that shoots BB guns, I have several 22s also).
Locally there is IDPA and combat (perhaps peculiar to this area) and if I drive in a 30 mile radius other options exist as well. I would love a 1911 in 45acp, but they're not cheap - IMO they're overpriced by about $300 - the difference between any 1911 and a similar 45 acp by the same company, any company.
Since I want adjustable sights and a long barrel that ups the price but it does limit the choice a good bit. But it still leave the caliber debate wide open.
I did a good bit of reading and have found that 45 is the most popular caliber for top shooters, and perhaps the most accurate, but that might be skewed by whose shooting it or that there are alot more 45's out there that are 'accurized', so to speak.
40 seemed to be the lease accurate and 9 in the middle. But I'm sure those results are debatable and probably vary more by gun than by caliber. Heck, most tests in magazines will show various ammos and you'll see some do 1.2" groups and another ammo do double that, or worse. So you could buy the most accurate handgun made and screw it up with the ammo you use.
If you dont' shoot much ammo price isn't probably that important, but if i shoot a season of local combat and never practice that's 1200 rounds. Now price per round makes a difference and 9 is the cheapest. It also will likely have the least recoil so that should aid accuracy, or at least reduce my fatigue a bit, and make it easier for wife and son to shoot (if I can talk them into competing).
Just read a blog on stopping power, where the guy collected data over 10 years about shootings and collated it all by caliber, where the guy got shot, how many shots till he stopped, how many perps didn't stop at all, etc. There wasn't all that much difference - a 357 came out better than a 22, but not by a whole lot - based on accuracy, total shots till the perp stopped, fatal shots, etc. The mouse calibers lost on one thing-while most still stopped after one shot if you kept shooting and shooting the 22 was the least effective on those that jsut wouldn't take a bullet as a definite NO! About 50% of all perps stopped with the first shot, regardless of caliber - he called is a psychological stop - the perp didn't want to get shot again. If someone pulled a gun on you would you stop or not, or would you decide based on what kind of gun he was holding? I bet the gun size, caliber, age - it won't matter, you'll stop cause you don't want to be shot.
I read a report that in NYC the average police shooting involved 4 rounds, total. So a magazine of 16 or 19 probably won't matter a whole lot, and almost all competitions limit you to loading 6 rounds then force you to reload as part of the challenge.
So 9 is plenty powerful enough to deter an enemy and cheapest to shoot a lot. My manhood is destroyed by not going with a 40 or 45, but I'll survive I suppose.