If I wanted a tactical shotgun...I'd pick the Benelli M-4 ...its gas operated / will be a lot more forgiving in terms of recoil if you want to shoot typically heavy loads of "OO" Buck or slugs...
Both pump guns and semi-autos have a significant number of moving parts...but I don't see "parts wear" as an issue if its a decent gun with usage under 250,000 shells...
------------
If I was not concerned about recoil ...I'd go with a pump gun...but I'd go with the Browning BPS like this model ...that comes with a 28" and a 20" barrel...
http://www.browning.com/products/cat...id=012&tid=273
On the BPS - the tang is on the top of the tang, easy to reach even with gloves on ....action is smoother than many other pump guns...cast on stock is neutral so it doesn't favor lefties or righties...just a good basic, solid gun.