I've eaten at Shaw's for their buffet Sunday brunch twice so far this year, once in each location, and it's outstanding. They have terrific breakfast items (thick-cut caramelized bacon, yummmm), terrific seafood items (crabcakes, chilled shrimp and crab), and terrific desserts (creme brulee, pot de creme). I wrote up a detailed report which you can view at www.chow.com/topics/364403#5927944 - the only thing that has changed since then is the price ($48) and they no longer have hot king crab legs, although they still have them cold. This is my favorite brunch buffet.
I have eaten at Reel Club for lunch and dinner and like it a lot, but have not been there for brunch. (I don't even know whether it's a buffet.) It's also worth noting that, since Shaw's and Reel Club are both Lettuce Entertain You restaurants, it's possible they may share ingredient sourcing and/or recipes.
Haven't been to Reel Club, but Shaw's is pretty good. It doesn't have quantity like most buffets -- it's actually fairly limited. But what it does have is fresh and very nice quality, and frankly for the price a downright steal. Sixteen would be my favorite brunch in Chicago. You can't beat the dining room/view and the seafood (full, cut, king crab legs) were worth the price of admission alone.