I have to agree with the "Go 9!" crowd here. Now, I also like the "big bang" single-action gun myself, but given a choice of the first design in .45 or the improved design in 9mm (the Hi-Power), I'd take the second.
If you really look at the situation, it will be evident the advances that the 9mm round and weapons bring forward on the modern battlefield. The times they are a changing. The .45 with 7 rounds, easily and rapidly reloadable, made for an upgrade over the .38 which was in prior service. It was suitably modern and effective for the time. What many folks seem to be getting sidetracked on is "stopping power", ie disecting the merits of one well placed shot. That's great but that isn't a description of modern warfare.
History has shown that the advancements that have led to decisive battlefield victory have included mobility and firepower. In WWI, the machine gun revolutionized warfare as did the train and the truck. It was not pretty. It was not about "disecting the merits of one well placed shot". It was about speed and superior firepower. Then in WWII, the aircraft and even faster, lighter machine guns and trucks created widespread panic and devastation throughout Europe at a pace that no one had imagined possible via the Blitzkrieg. Horrifically, both the most advanced automatic weapons and vehicles were German possessions.
We can then have a look at more modern wars such as Vietnam and see a transition from the awesomely powerful, yet brutally heavy to carry M14 Garrand to the M16 Colt. One reason was to be fortunate enough to be able to carry a much lighter, higher firepower weapon. Perhaps the most overlooked issue is the weight and bulk of the ammo that is required to support these weapons. If you can imagine how important it would be to have the most ammo at any one position in combat, you're begining to see the logic. Not that Vietnam was a victory, nor was it a real testiment to mobility, it did prove that a lighter weapon with greater firepower and the ability to carry more ammo was not a bad thing. I'll not argue that the AK47 was not very effective, only that its ammo was very heavy by comparison and had the war been on other than its own turf, it would have been more difficult to support.
So the sidearm really didn't change much as in both WWII and Vietnam, the objective became increasingly to increase distance and speed of attack to minimize casualties and to inflict the greatest on the enemy. Historians will note that this was not always to our advantage due to the equipment and logistics of it all but at least we progressed to carrying more suitable arms for the shots that were more decisive, the distance rounds. Then in the most modern conflicts, we have seen mobility and firepower become even more important as we have evolved into a predominately air supported army. The army that the air forces are supporting though has increasingly been subjected to urban combat. This makes the smaller, faster cycling, and greater capacity weapon the more desirable. It also makes the sidearm an actually used and important arm.
I believe the US military did perform effective and intelligent trials to reach the conclusion that the higher capacity weapon with lighter ammo was the best choice to meet its current challenges. It also allowed us to save some money per round and as a side benefit, that's fine with me. I'm most concerned with saving the lives of our soldiers.
-Bill
now my day is complete. 
