The 12 gauge....
Next best the 16,
The 20,
Then the 28.
Simply put, the more lead you launch the better the odds if properly choked for the situation. However both the 20 and the 28 are easily adequate for Grouse.
The advantages of the sub-gauges is the lighter carrying weight but not recoil reduction per se. A 3/4 oz load at 1300fps recoils less in a 7 1/2 lb gun than a 5 1/2 lb 28.
Back to the meat of the question, 20 or 28 with no accounting for ammo cost or availability.
As much as I love the mystical 28, I have to go with the 20 as the overall winner. There are more loads to choose from, letting you extend your species list to Pheasants, Geese, Ducks or Deer. By far you will find that the 28 will be heavier than the 20 in the same model gun as they use the same receiver for both and the 28's barrels will be of thicker wall dimensions.. unless..... you are willing to pay big bucks for a gun that has its receiver scaled(smaller/lighter) to the 28.
With all the logic above though, everyone needs to experience the mystical 28 in the Grouse woods.