Both cars are extremely well built and of similar quality. The name Porsche might sound more prestigious, but that would be an irreleveant decision breaker (to me at least). If looks is of some importance, no doubt the GC takes the price.
Very few customers have driven the GC yet. So I guess I have to base my comments on the experiences I have with the F12/13, and the reviews of the GC.
A basic difference is that the Panamera is of sports car blood, while GT-blood runs in the BMW vains. The Panamera rides and handles as a (too big and heavy) sports car, and do need the V8 to have credibility on the engine side. But even so it does not convince as a sports car, and there are too many compromises for being a GT car (I admit these are complaints on a high level, and I could live with them
).
The GC has other talents. Fast, comfortable, and built for those remote destinations or having fun on your favourite country road. Not built for the race track. But neither is the Panamera, and that's why I think the GC is a more honest car. A shortened 2+2 seater Panamera would be a better concept from Porsche imo.
Having the choice between the two cars, I would pick the GC. Without any doubt if you talk about the 6-cylinders. If the choice is between the V8's I would make a thorough test drive and listen to my gut feeling as well.
Some of us have a blue and white heart, but that would also be an irrelevant argument.....