Subsonic refers to the velocity of the round, in that it travels less than the speed of sound. The "bang" won't be different, but if you use a suppressor it'll be a bit quieter because the bullet won't have the "crack" that supersonic bullets have when they leave the barrel/suppressor.
In general, hollowpoints won't overpenetrate, but XTP might because they're less likely to expand than other hollowpoints. Refer to the Lucky Gunner test in swsig's post. Granted, XTP will probably penetrate less than FMJ, so if those are your only 2 options, XTP would be the better choice.
It's always a good idea to run several rounds of whatever round you want to carry through your gun to make sure it'll be reliable. Some people recommend 100-200, or more, trouble-free rounds through their guns. At the very least, I like to run at least a few magazine-fulls of FMJ followed by a few magazine-fulls of JHP as a bare minimum. However, given the current ammo availability problems, it can be difficult to get enough carry ammo to properly function test your gun and still have enough to have in your gun to carry. Sometimes you gotta do what you gotta do, even if it's less than ideal.
Just my opinion.
