Do like I did... get a Ruger 10/22 and sink $400 dollars into it.
All joking aside, I'd probably go for the Walther. The name says a lot. At this time, I still see the GSG-5 as a novelty. And because of the import status, you will have to deal with 922(r).
I've been down that road with both Saiga's and straight AKs. While its not that difficult once you get the hang of it, I suspect that it would be a pain with the GSG-5. The AK has a LOT of aftermarket parts available from domestic manufacturers where the GSG-5 has none as of yet. I suspect that this will be the case for some time.
But really, it comes down to what kind of gun you want to end up with and what you want it to do.
-- John