I don't think there is too much difference in them. I wouldn't call it faster, the Gustav is supposed to be better for killing foot soliders. Don't quote me too hard on that, It's just something I read and something I've come to believe after swapping launchers myself
The RPG is supposed to be better for destroying tanks and the guided AT4 is for destroying Helos, but I struggle using it. It's probably the same reason I can't use the stationary launchers, either.