I've shot my share of doves and perdiz with both the 20&28 gauge, perdiz are not quail, but comparable on the 20 vs 28 debate.
I would think there's a slight difference, which is not really an issue when hunting for hyper-abundant perdiz in SA. That small difference will be much more noticeable if hunting in a place with less or very few birds, or/and with much hunting pressure, where every single shot counts....
For doves I have found a really advantage in the 20 when there's lot's of wind.
This being said, I've shot the 28 gauge for doves 90% of the time.