How could one be better if they do the same thing?
One you shoot from your hand. Obvious enough
One you shoot from the underbarrel of your gun. Fine example of convenience right there.
So put simply, If you have a gun, but don't want a grenade launcher on it, but you want a grenade launcher, buy the standalone.
If the situation is reversed, and you don't want to carry extra.
Put one on the gun.