I would personally prefer .45, but I have no military experience. And the enormous cost of a conversion, coupled with the inertia fighting any change even if real evidence of the inferiority of the 9mm existed, would make such a change take a number of years.
The son of a friend of mine saw considerable action in Iraq. He is not impressed with the combat effectiveness of the 9mm. He is likewise not impressed with the M4, the short barrel doesn't give the 5.56 the velocity it needs. He says that the 7.62X39 fired from AK variants is much more effective in the type of fighting that he saw.