Depends on what you want or think a GSD should be. If you want to really support the preservation of the breed, as it has historically been, or if you have the need for a dependable working dog, then I would say no show lines.
European showline dogs don't have the perverse slopey back that the extreme American lines have, but extreme European showline dogs have nasty roachy backs. And yeah, they are held to a higher working standard than American show line dogs (which are held to none), but that doesn't mean that they are actual working dogs. Possibly, if you never do anything with your dog and you never encounter a situation which requires him to perform, a European showline dog will suit your purposes just fine. You could end up with a European showline dog that does measure up. It really comes down your vision of a GSD. Mine was one of a determined, athletic, driven, rock-solid nerved dog with a "natural" appearance... not artificial/manipulated looking. That was for aesthetic reasons (I also love sable, reminds me of a wolf), but even if I wasn't attracted to that "type" of dog, dogs that have those highly artificial structures tend not to be sturdy as more primitive ones. Obviously, I'm pro working line.
American working lines? Haven't really heard of that. Any working line dog you will find is going to be "Czech" or "DDR" or from some sort of European origin. These lines are taken and bred in America and some people don't think that Americans can match breeding pairs and cull properly, so they go for imports only. That might work as a generalization but of course there are good working line dogs bred in America, you just have to know how to find them, which could prove to be tricky. Also, importing from Europe isn't foolproof either because Europeans, knowing that you don't know much, will send you their eh-eh dogs. So here you've gone to the expense and trouble of shipping this thing across the pond and it isn't any better than your working-line dog from the classifieds.
I wouldn't worry about importing a dog no matter what the lineage. The most important thing is not the line, it's the breeder. So you want a working line dog, well, knowing the line is the starting point, but the quality of your dog will really come down to the breeder. If it was as easy as contacting a breeder in Europe and having the perfect dog sent over, that would be nice, but in reality you'll have to breeder shop in Europe just as in the USA, so you may as well save the expense and hassle and find a pup here.