Bob:
I've played Kingsley roughly 10-15 times and have only 2 rounds at Greywalls. I don't know if that affects my analysis in any way, other than I think I have a lot more to learn about Greywalls and thus an itch to get back up there to play it again.
The main difference in the courses is the severity of the site at GW. Its a rugged course, certainly walkable but definitely a tougher trek than KC simply due to the overall elevation changes of the site. From a fun factor, the two courses are about as good as it gets. You can play the ground game at both courses, with KC offering a bit more quirk in places while the prevailing theme at GW is negotiating the elevation changes.
GW definitely offers a special feeling, but perhaps for different reasons than KC. There's something about the scale of the course, the wide cascading fairways, the greens perched among outcroppings of rock, the views over Lake Superior and the sense of isolation that makes it feel like no other course I've ever played. In its own way, Kingsley is also a unique setting.
As I noted, I feel like I know KC better than I know GW. I have the sense that even with additional plays in Marquette, the course wouldn't get old. Right now, I'd probably have a 6-4 split in favor of Kingsley and would guess that would be the same even if I'd played each 20 times.
Hope this helps. I guess the short answer is that even though its a long way to go to play golf, getting up to Greywalls is definitely worth the effort.
Sven