You should play Greywalls at least twice - the first time around will be a learning experience and there is an awful lot to take in. The views are incredible on top of the golf course, so you'll want to have the second go around for more than just the quality golf.
It is true that nine holes of the Heritage were designed by Langford & Moreau, though those nine are mixed in with the nine added by David Gill in the 60s. The current numbers of the original Langford (in the order they originally played) are 10, 11, 7, 8, 4, 5, 6, 12, and 13. It is a very enjoyable course and the superintendent, Craig Moore, really gets it and has begun to slowly bring back more of the original design. He's already pushed at least one or two of the green edges out to the original size, making use of the high shoulder settings. The Gill holes are also nice, particularly in the challenging topography of the valley behind the golf shop. Holes 14 through 18 are especially good.
I haven't played Sweetgrass, so I can't really comment on it, only to say that it's something I hope to play in one of my trips back home. Timberstone is a resort course with a fair amount of eye candy and some incredible views, but the style and golf are entirely different than Greywalls. The appeal of packaging them together is that all three are very different and in a large group, all three will be the favorite of somebody.
A few other UP courses to consider:
Indian Lake GC in Manistique (near US-2). If you're headed to Marquette from the lower peninsula and have some extra time, it may be worth a stop. The original nine is very, very cool, and the back nine was added by Mike Husby about 20 years ago. It is usually a ghost town.
Iron River CC in Iron River (near the Wisconsin border). Coming up through Wisconsin, this would be worth a look. It's a nine hole Langford design, with two sets of tees so that it can be made to play as an eighteen holer. The usual Langford features are easy to spot, though as has been the case with most of his courses, they have softened a little and/or been altered over the years.
Pictured Rocks GC in Munising (near M-28). Although it sadly offers no views of the amazing Pictured Rocks (which ironically can be seen from the 1st tee at Greywalls some 30 miles away), it is a fun little local track with some good holes.
Pine Grove CC in Iron Mountain (not far from Timberstone). Some very, very good holes on an underrated track that is always in great shape. Another nine holer that saw expansion 20-30 years ago, the flow of the course is remarkably consistent and for generations it was considered the best of the UP. If you're going to spend some time at Timberstone and check out the town of Iron Mountain, it's worth playing here as well.
All four of those are also incredible bargains.
For my money, when I go home to Marquette, that's where I stay. The golf is unparalleled in the region and the town is easily the most attractive in all of the UP (though I may be a bit biased). It's got a great downtown district right on the shore of Lake Superior, neat restaurants and boutiques, a couple of great microbrews, and no end to the list of outdoor things to do. Climb some ancient volcanoes, kayak around Presque Isle, run the endless miles of beachfront, or just poke around town. And when you want to feel like you're in an entirely different world, head out to Greywalls. Marc Gilmore and his staff will take excellent care of you and you'll have a blast.