I used to think that Levi's 501s and 505s were the same thing aside from the fly (501s having a button fly, and 505s having a zipper). But after looking more closely, I get the impression that 501s fit differently from 505s. Can anyone tell me if this is true? I've never had 501s before -- always 505s. But I'd like to switch to 501s. How is their fit different, if at all?