The fact that the one ounce 20 has a longer shot column than the one ounce 12 cannot be disputed. The significance of this "imbalance" in the field is arguable. Assuming equivalent chokes you would not be able to demonstrate a significant difference on the pattern board unless you were willing to shoot upwards of 50 to 100 patterns. This is to say that a given 20 gauge pattern might look better than a given 12 ga pattern.
The other corollary myth is that every pellet added to a "square" payload ends up outside the useable pattern. This is the purest of bunk.
There is some basis for arguing that larger bores handle large shot sizes better. If you need to throw 1 3/8 oz so you can make 50 yd opportunities you might want #5 or #4. The 20 is not optimal. Neither is the 6lb 2.5" 12 ga Brit gun. But the one ounce 20 handles #6 just fine. #6s are adequate to 40 yds for about any upland purpose.
If you like a particular 6lb 10 oz 20 I say go for it. And if you believe that the weight will help you take more birds so much the better. I once foolishly passed on a 7# MX28. Guess the traditional British rule got the better of me that day.
Different people may see the weight vs shootability compromise differently. There is no single correct answer, British traditions notwithstanding.
Sam
