+1 to whatever the unit carries
In the real world, two points:
1) You don't get to take your personal sidearm, except in certain elite units.
2) Even in those, there are usually only a few models the guys go for, so ammunition and parts have at least some commonality.
Even in the theoretical world, why go for a monster sized gun like the MK 23 when it's just a backup to a rifle, and when you will probably be packing a lot of other equipment around? The IBA alone will run in the ballpark of 30 lbs; add to that a kevlar helmet, pack, camelback, ruck, etc...
So, I want whatever is commonly available, and preferably light and not too bulky.