I am not familiar with the GD breed, but my sister had 4 GSDs and we even found two on the side of the road back when I was very young. They are very smart dogs. The one we had in the house was not destructive at all. Many people think of GSDs as vicious because of the police dog aspect of them. None of the Shepherds we've had were ever vicious. Not even the ones we found. In fact, one of those was a scaredy cat (appropriately named Boo).
I personally love German Shepherds and all of the Danes I've met have been very sweet as well. I would like to think that mix would be a great (big) wonderful dog.
But every dog is different and has their own personalities. Visiting with Rex is a great idea. I think you'll know if he is right for you or not after visiting a few times.

Good luck! And if you get him, please come back and post some pictures of him.