FBI can use HPs, and uses sidearms much less than military members in combat zones.
The FBI uses a sidearm as daily carry, the military uses the M16 and variants as it's primary firearm for personal defense. In the average combat unit, very few use the sidearm as their primary weapon. Most of the time it's issued for personal carry outside a combat zone.
Even the MP's rack the M9 and use M16/M4's in combat. So, on a daily basis, the FBI is more likely to have to use their sidearm than someone in the military. They carry on duty - the military only carries in a combat zone, at home, not at all, for months at a time. Soldiers aren't issued weapons unless they are scheduled to shoot. It's a highly controlled exercise, too, and on post, soldiers can't carry personal weapons during the day, either. Exactly why the Ft. Hood shootings were so successful - it's as bad on post as in a school or at the Mall. One big victim zone, and it's not going to change.
The FBI changing back to 9mm just reflects the reality that it's not the same ammo that lost the Miami shootout, or even that the incident should have had the effect it did, and that the results are more fully understood. As for the military looking into replacing a service pistol, it's mostly issued to officers in units, the MP's carry it as a peacetime duty gun to signify their authority, and very few ever draw and shoot it outside the range - because it's almost nobody's primary combat weapon. Entirely why the few who would get to purchase exactly what they want for that limited number of situations.
The military will likely choose a weapon more based on political and social constraints than efficiency, the Beretta was chosen because we had a refueling port in the Med in Italy, and a parachute regiment stationed there. Plus - the trials were held at a time when Glock wasn't yet ready to submit, being in their infancy. Glocks are now purchased and issued to some units.
Being a service pistol, it will be large frame double stack double action with redundant safeties and a 25 year service life expected. Entirely off the reservation for the FBI, who would likely prefer something smaller, potentially slimmer, who carry under jackets or concealed, and have fewer institutional traditions to overcome.
