For new wp designers, I’d suggest CC. Protect yourself and protect your content. At least, it’ll stop thieves from blatantly ripping your theme and promoting their own version on public databases. Once you’re well known, go with GPL.

If you worry about growth and linkbacks, there’s really no difference between the results of GPL and CC. Most bloggers don’t remove credit links because they respect the effort put into developing themes.

For the ones that do remove credit links, they’re going to do it regardless of your license.