I have both semi and pump, and to be honest, given the recoil of a 12g, I can't get aimed shots out noticeably faster with the semi. I can get shots out faster, but not aimed shots.
And that isn't just me. If you train a ton, and I'm talking 100's of rounds a week at minimum, you'll eventually be able to get them out, aimed, faster with a semi 12g. If you practice with a pump, the shoot-rack-shoot becomes fluid quickly, and you can do it at the same time as you're bringing the barrel back on target.
Pumps are, generally, simpler and easier to maintain, and require less of it. I rarely clean my pump - at the end of the year before putting it up for the winter, and then wipe off the excess oil in the spring and shoot all spring, summer, and into the fall. My semi gets taken down and cleaned after every trip it makes to the range.
Pumps win for reliability. Always.
Pumps are ubiquitous. If you go with "one of the two" pump guns that most people end up getting sooner or later, you're more likely to be able to get spare parts down the road. Semis are far less common, and there isn't one or two big market dominating semi-auto shotguns out there. So if you're semi breaks down in a Mad Max world (or zompoc, SHTF, the illuminati shape shifting lizard alien men that are really running things or whatever floats your boat for needing a SHTF gun is), you're basically out of luck and holding a club.
Pumps will tolerate any ammo that fit in the chamber and you'll be able to cycle it. Semis do not. And by any I mean any. I keep a couple boxes of 12g flares and 12g bear bangers around. Throw a couple of each in my pocket when I'm wandering in the interior. Either one of those is far more likely to save my middle aged @ss in an emergency than actual shot is. Running a flare or banger shot through a semi falls under the category of a special kind of stupid.
Capacity: Because of the laws here, you can set up your capacity in advance with a pump - no limits. For a semi, you're limited to 5 rounds, and if you even get caught with the parts for making it otherwise, you're in for a potential 3 year free room and board at club fed.
Tube or mag: Tube. Again, it's about simplicity and reliability, and tubes win on both counts. Also, for my bush load, I only load 3 in the tube and leave the chamber empty, even though I can load 4. While I have yet to actually need to put it into practice, my thinking is along these lines: two slugs in, then a 00 - so the 00 comes out first in an emergency when I don't have time to aim as well as I might like, then follow up with 2 slugs. Any problem (bear) I come across that needs more than that, well.... Bears are faster than you think, the odds of being able to even get off all 3 of those shots are slim. That leaves room to quickly throw in a banger and rack and shoot it if the bear is closer than I would like it to be, but far enough away I don't want to shoot it needlessly (in the 75-125 yard range... Further than that, just leave it alone and hope it wanders off). Under 75 yards, I'm racking a live round and hoping it doesn't charge.
Someone will come along and pick apart everything I've just said, point by point. Have at it, always willing to listen to an idea that might be better than my own.
