Those who actually shoot the 28 and have hunted with it know better than to compare it with the 20.
Ammo for the 20 can be bought in 4 packs for $20-something dollars.
Quality 28 ammo is $10+ per box. Import ammo can be had for around $8, almost twice as much as 20 ga fodder. The one ounce "magnum" 28 is almost $20 per box, and does not deliver magnum performance.
I have shot the 28 for over 50 years, and hunted with it when I was younger. It DOES tend to be disappointing in the field, unless discretion is used to pass up longer shots.
With this said, some one will likely write about $3 per box ammo and 50 yard kills with it.
With this said, my 28 ga O/U's have 30" and 32" barrels, and are a lot of fun on the Skeet field and sub-gauge Sporting Clays course.