Some perspectives that maybe haven't been discussed here yet:

Chest rigs are easy to draw from while in a vehicle. Hard to draw from while prone.

If your wearing a chest rig / vest with a standard belt holster, sometimes the vest can get in the way of drawing the weapon.

I personally don't like thigh rigs because I don't like having my weapon that low. To me its odd while runing and feels insecure. And when drawing it takes just a fraction of a second to get the wepon on target.

Oh, and the worst thing about a thigh right is if your leg gets blown off you can't draw your pistol to take out the guy with the RPG that just pwnd you....think about it.