Right radius and balance
The longer the sight radius, the more accurate the gun can be aimed (though the difference between a 2" and 6" in ACTUAL accuracy is negligible, the ability to AIM is heavily influenced by the distance between the front and rear sights).
I have 5.5", 6" and 10" barreled (otherwise identical) handguns. The 5.5" and 6" are virtually identical in accuracy, but the 10" shoots almost as well as my carbine/rifle in the same chambering. However, the 10" is awful to shoot, as it is too muzzle heavy to hold well.
Some people like the balance of a slightly muzzle-heavy 6" to the more centrally-balanced center of gravity of a 5" or 4" barrel. So, I opine that it will depend on your particular style of shooting. How you hold the gun, your eyesight, etc.
I know this is not a definitive answer, but it gives you my opinion of the factors that deserve your attention when you make your choice.
My choice in GP100 for range use? 6" most of the time. (I have both 6" and 4" from which to choose).
Lost Sheep