another brush question

I just can't find it on the forum or the internet. What is the difference between all the Purdy Models. Some are self-explanatory, but I'm mainly asking about the difference between the Swan and the Sprig. Both have the beaver-tail handles, and "square shaped" ferrule and bristles. my guess is the sprig is a little stiffer than the swan or something like that? any input? Even the Purdy website couldnt help me. Also, I was just reading the old venetian plaster thread. I would love to practice this art at home, are there any good books on building small lath, and application? What are good trowels to use? Lowes ok? Ive bought plenty of bad brushes from there, so I don't know. I don't do any drywall work so I don't have any of those kinds of trowels. Thank you for my future replies. happy weekend