Pretty hard to tell the difference, but when I was hunting them I always thought the prairie chickens had more black on them. If you see them on the ground walking around you can tell the difference easier.
Unless you're hunting grouse in the prairie chicken zone (which you can't, anyway), you shouldn't even have to worry about it.
On a side note, I did get a letter from the NDGF along with my prairie chicken tags that grouse numbers are up quite a bit up in the chicken zone, and they want as many sharptails harvested as possible as they may out-compete the prairie chickens for food/habitat.
On the downside, that time of year it will be hard getting close to either!