If you want to know what the fastest tire width is, just look at what the pros use. In the Tour de France, it's 23 or 25mm (23mm used to be the standard, but 25mm is becoming more and more popular). On rough terrain, such as Paris-Roubaix, it's 28mm (or 25mm with wide rims).

If you have a choice between 25 and 28, go with 28. You won't notice the speed difference, but you will notice the extra comfort. You'll appreciate that choice at the end of a long ride.