While you might have already made your choice, I’m going to chime in.
The first two considerations, in my humble opinion, are quality and fit. Valid arguments can be made either way as to which one comes first. Hunting is more than meat collecting, and a fine hunting gun can be a treasured companion for life. A hunting gun is one of the few personal items with which we develop a close bond. It doesn’t have to be wildly expensive to be one’s pride and joy. Quality begets reliability. A quality hunting gun may never require gunsmithing fees or ‘warranty claims’ in your lifetime. A gun built before everything got cheapened and cost-cut would be my first choice. The second-hand gun rack (and the EE) is your friend. I should also say that fit on a hunting gun isn’t the same as for a competition shooter. Most hunting guns fit me well, perhaps I’m just lucky.
Following closely on fit, is handling. Difficult to define, but it has to feel right for the kind of hunting you want to do. A lighter weight gun to carry all day, and a good barrel length to help swing on flying birds. Chances are that a quality used gun will have a proper field-length barrel already. Everyone’s body is different, so handling a variety of guns can help determine what works for you.
The two semis that I have are a vintage Browning Auto-5 in 16, and a Browning Double Automatic in 12. Both came out of the FN factory in Belgium, and are extremely well-designed and built. The Auto-5 cost me less than $300, because it takes 2 1/2” shells and the ‘market’ deems such guns as troublesome and un-sellable. It’s an extra effort to source and stockpile the ammo, but 1oz loads are perfect for any kind of bird hunting I will ever do. The Double-Auto does not care what 2 3/4” or 2 1/2” shells go through it, it works every time. I prefer 7/8 or 1oz loads, easier on the body and gun, and kills birds dead. If for some reason a third round is needed (rare on grouse), the Double-Auto loads and chambers a round in the proverbial blink of an eye.
I choose the gun before I consider the gauge. Every gauge works just fine. I don’t have personal experience with the 28, but if the gun is pointed properly, the bird won’t know the difference…
I haven’t mentioned choke, because that is not a consideration for me. Proper mounting, aiming and timing will kill the bird, not choke. If one’s main worry is an extra single-digit percentage of lead within a circle, I would argue the battle is already lost, and more effort should be made to shoot well. Versatility in choke dimensions does not fix poor wing-shooting. I don’t foresee ever owning a gun with removable chokes. I can understand opening choke if one shoots steel shot, but that’s about it.
Yeah, I’ve gotten grumpy in my old age.