From my experience, I’d say pulykamell is right about the frequency. Just out of curiosity, I did Google searches and came up with the following:
[ul][li]shitload: 39,400 hits[/li][li]shit-load: 17,900 hits[/li][li]buttload: 8,820 hits[/li][li]assload: 5,410 hits[/li][li]butt-load: 3,940 hits[/li][li]ass-load 1,500 hits[/ul][/li]So that backs up personal observation, though I’m surprised that assload/ass-load has so many hits since I almost never come across it.
I’m not entirely sure what you mean by “magnitude,” but I take that to mean the intensity, or forcefulness of the expression. If that’s the case, I agree with your friends that buttload is the lowest magnitude and shitload is the highest magnitude. My reasoning is that butt is the least crude and shit is the most crude based on how acceptable they are generally considered. Just look at whether they are used on TV: I don’t think there is any problem with saying “butt”; “ass” is less common, but not rare; “shit” is fairly rare.