I've seen some very talented screen printers in terms of art, and in comparison to sign makers, the screen printers I've seen are much better because they have a broader audience that requires a more flexible talent with art versus sign makers have to do a lot of boring corporate looking stuff on a very regular basis.
On the other hand... I haven't met very many screen printers who had the potential to go further than screen printing. Whereas I know a lot of sign makers who are inclined to figure out how to do pretty much anything on their own.
Really, how many screen printers fix their own computers and printers?