Matt --
I have played both and agree with your thought that they are very close in overall quality, even though the character of each course is quite different (because of the vastly different properties involved).
First, a disclosure: I have taken advantage of the amazing deal for Marquette's national membership. Marquette is a neat city that my family and I enjoy visiting. (It has the best playground I have ever seen.)
Each course shares the fascinating appeal of having the golfer leaving the 18th green wanting to proceed immediately to the 1st tee, as it takes many rounds to begin to "get" each course. With each round you realize a new way to play one hole, etc., and that wants you to rush back out. On each course you look forward to playing the next hole.
I agree that the 1st at Greywalls is a contender for the best three-shot opener I know. The prospect from the tee wants you to play golf immediately, and the green is outstanding.
Other favorites (among many) at Greywalls include the 9th and 14th. The approach to the latter with the angled green is superb. The 14th will likely be a sleeper hole, as it may proveto be the hardest par on the course.
What may ultimately determine where Greywalls fits in the world golf picture is how holes 15-18 play. After just a few rounds there (and then the 16th and 18th greens were closed), I can't say. The 18th is a hole where you just need to forget about what par is and look at it as a big hole spilling down the hill to a green that looks deceptively inviting from 220 yards. The transition from the hill to the open green is intriguing, and the green looks to be very interesting, with a central hump that I imagine dictates where you want to place your second (left or right, to be on the same side of the hump as the hole). I'm looking forward to playing that green.