I see there is a superlong posting about iron sight.
The simple version is that if you go out and buy a set of BUIS off the shelf, the closer the front site is, the fatter the front sight post appears and the more it takes up space. It is difficult to put down accurate shots when the front sight post is fatter than the target. That is why carbine (with the front site about 8" away) was straightly a close combat weapon) until someone put the first gen Aimpoint on it, and then the M4 became a viable 500m weapon once ACOG became a general issued item.
The major advantage ( for sports shooting) of the longer handguard is that it allows you to grab the rifle as far out as possible. Think about this, you hold a sledge hammer with the support hand as far out as possible at the beginning of the swing so you can use less power to move the hammer hand. It will use more energy with less control to move the sledge hammer with the support hand close to your holding hand. It simply give your faster first shot from ready, better recoil control and better control in moving from target to target.
On the otherhand, if you are loading up your rifle with laser and illuminator, depending on the setup and the weight of your kits, you may need to use a vertical grip and a dual switch. In that case 9" is probably all you need. 12" is just a waste of space because you don't really want to put all the weight any further out.