The sole difference between 10, 12, 16, 20, 28 and 410 gauges is the number of pellets in the shell. Each of these gauges should pattern the same, with same chokes in each gauge. The advantage of the larger (smaller numbered) gauges, is that they hold more shot, ergo, produce a more dense number of pellets into the pattern. By the way, the velocity should also be the same, or very similar. A 20 Ga is an extraordinary gauge for upland hunting! I use a 12 Ga, with 2 3/4", 1 and 1/8 ounce of 4 shot, at 1,200 FPD only for the extra shot, over the 20 gauge with 1 ounce of 4 shot, at 1,200 FPS. If I were to purchase a 2nd shotgun, it would be a 20 gauge. That is in fact, what I wish I would have bought in the first place.
Edit to add: Read this textbook: Modern Pheasant Hunting, 2nd Edition, Steve Grooms, Stackpole Books, ISBN: 13:978-0-8117-3227-7.
My wife and daughter bought me this book for Christmas this past year. Later, I bought my Wingmaster 12 guage. I went with 12 gauge because I wanted extended range ability. Read the book and make an informed decision.
Doc2005