Free: Contests & Raffles.
I have both as I may be a gun hoarder. I will say that ammo is cheaper and readily more available for the 20. It also has more variety out there. It is possible to get both guns in the same weight. I have a 20 that comes in less than 6 lbs and my 28 is listed at 6 lbs. Hunting loads for the 28 I think are listed at $18 per box with limited variety. 20 gauge is way more cheaper. Having said that my next gun will be a Browning o/u in .410.