RE: Beretta AL391 or Silver Pigeon???
Light loads might not operate the gas op recoil. I am a big fan of the o/u, but would likely opt to the 20 vs the 28. What is the use of the gun, hunting? Sporting Clays, trap???? Get a Sims Vibration Labs Recoil pad and have it installed or DIY. These are great, I put one on my .338 and it reduced recoil by 50% or better.
Either is not a bad choice, I just parsonally don' t like the berettas. If you want the auto, I' d look at the Benelli' s. More reliable w/ lighter loads and IMHO, easier to shoot. Like I said, JMHO!