There is less muzzle rise using the heavier buffer. If all you do is shoot paper stationary on a range, it won't matter. However, if you are running drills, believe that a Pact timer is both your friend and nemesis, or use it as a weapon, you will find the change to be evident an dmost likely appreciated.
If you run a carbine with a standard buffer, and then with a 9mm or one of the heavier buffers and can't tell the difference, you are very out of touch with your weapon and need to get in more trigger time.
Its worth pointing out that any time you make a change, you need to ensure that your weapon still runs like a champ with your ammo. Weak ammo may not cycle as well if you make this change, then again, it may be fine for you. Either way, check it before you rely on it.