Yes, "www" is an alias for the "public_html" folder. It doesn't matter which you use, although the convention is to refer to public_html. Just remember that you have ONE copy of your folders and files, not TWO separate copies! If you erase a file from www, it's gone from public_html too.
If Dreamweaver doesn't have any run-time component on the server like FrontPage/Expression Web does, I wouldn't think that the two would interfere with each other. You can always try publishing to public_html (your site root /) and see if both still work, and if they don't, move the new Dreamweaver stuff to a separate folder. Hopefully you won't have to do that, but who knows (I don't use either product). The big thing would be to make sure you don't have different files with the same name produced by both of them.