There is going to be a lot of personal preference on this one. I have one on my ar as well as on my KSG. They both perform different functions for both guns. On the KSG it does nothing that provide me with a locator for my hand. I was grabbing the pump handle to close and not getting a full pump action when I ran that gun.. added the afg and never had the problem again. On my AR-15 on the other hand I find it much more comfortable and ergonomic to shoot with the afg. I doubt it effects my accuracy with the gun. just make it more comfortable, as well as providing me with a hand locator so that I am getting the same grip every time in the exact same place every time. And when running and gunning this CAN help my accuracy for quick shooting.
I've never run a verticle grip, but have nothing against them.. Just remember, they arn't ment to be gripped like you would grip a pole...they are ment to be half held with your hand up on the handguard....acting as a locator. Personally I prefer the AFG, it takes into account the natural angle that my hand wants to be at anyway.
You can't really go wrong with either one as long as you use it correctly and place it correctly for your body type.
