Sorry to disagree - the pump is a compromise and a poor one at that........I'd go with a gas gun for greater reliability and less recoil
I'm a gunsmith who actually started out working on 1100's to keep them firing at the trap range. Word got around I knew how to work on shotguns, and particularly semi-autos, next thing you know, I'm fixing rifles and pistols, too. But I digress.
I disagree with your comment; A beginner should definitely start with the simplest system (pump) first, develop skills with that, and maybe then think about changing to a semi-auto, if that's what they want. Beats being handed an old 1100 that won't fire three rounds at ducks without jamming and being told, "Well, fix it, then!" Sink or swim is no way to learn, especially with a fighting shotgun. I never set that 1100 up for HD/SD, and I'm sure glad I never had to trust my life to it. All I lost because of it malfunctioning is a few duck dinners. I bought an 870 shortly after my 18th birthday, and gave the 1100 back to my Dad.
The small amount of recoil reduction from a gas system is not enough benefit to risk failure from lack of/improper maintenance by an inexperienced user. Now if you are talking an experienced, motivated, well-trained operator, the action type will be of no consequence; they will have the training to deal with any problems; yet still, the vast majority of users of shotguns in a fighting environmant choose pumps, and exactly for the reason of reliability. Also, the ability to fire munitions that will not cycle a semi is another good reason.
The only malfunctions I personally have ever had with an 870 is from operator error, i.e., short shucking. The only other malfuntions I have ever seen on any 870 besides that is the infamous 'double feed', or shell stuck between the carrier and bolt, and this is caused by an extreme lack of maintenance in not replacing the feed latches before failure. (or not pushing shells far enough up into the magazine, again; operator error)
For a semi-auto for someone who has mastered the basics with a pump, I would recommend a Benelli M (1,2,4, 1014) before any gas-operated shotgun. Indeed, I would recommend the A-5 or Rem. M11 before a gas operated semi. The Saiga shotguns would be about the only gas-operated shotguns I would recommend, and that mostly because I prefer AK's to AR's (Again, reliability), and the added capacity. While
extremely familiar with the 1100/11-87 systems, I cannot recommend them, or any other shotgun that relies on a fragile O-ring to function. (That ring in my wallet as a young man was not from a condom...) I won't go into what a PITA it is to scrape carbon from the port and rings on an 1100/-87....
I don't know much about shotguns, but i do know two things. Listen to these guys who tell you that a clays/trap/skeet gun is going to be very different from a defensive gun. Especially if you like a pistol gripped stock (NOT pistol grip only). These stocks are great for what they're intended, but very, very poor for wingshooting.
I'm not sold on that. I routinely shoot the same average with my SpeedFeed III pistolgrip 870 as I did with the Wingmasters or the TB I shot trap with. 23/25 either way. 16 yards or handicap, doesn't seem to make a difference.