Chest rig mounted Well You draw it and sweep yourself with a loaded pistol. KevinB talked about this before. Also if you don't need to wear your vest gonna have to find a place to put the pistol in being cannot leave just a pistol in a vest...What good is it then.
Shoulder holster are great being well you don't need to wear pants/shorts with belts, can wear running shorts etc and throw it over anything. Great for on base, But not great for a combat holster. If you hit the button by mistake and your pistol could fall out, I've seen that a lot. Also If it is to be loaded, not great walking around laz'ing people behind you, or when you draw sweep yourself with a loaded pistol. Then again should not have a mag on it on base anyways. Also when you have body armour, Plates, Tac Vest on your gonna find that arm restriction is maxed and might not even able to grab your pistol out of the shoulder holster.
Thigh Rig is your best bet. 6004 or the Serpa.
As a Driver I could not wear my vest while driving being the tight spot I had. SO I had my pistol on a Thigh Rig, TAC Vest on a Quick Release buckle behind my seat next to my C8.