I haven't read all posts before me so some of this may be re-iterating whats already been said.
A dependable, proven Semi would be better IMO. The reason you see so many pumps is their reliability is generally inherently better because you aren't relying on "automatic" motion for function. Some are recoil operated and others are gas operated. Different loads cause the gun to cycle differently, the way you hold it cause it to cycle differently. If you get a low powder charged round the gun will function differently, in some cases not functioning properly. Pumps will fire any loading from barely getting the load on target to magnums.
The other reason you see alot of pumps in HD roles is cost. A gun that one hopes to never have to use (except for practice/fun) you can get a good HD pump for under $400. The Semi I would deploy in that role exceeds $1000.
All that being said I prefer a pistol to a shotgun for initial HD contact anyway. Pistol fights me to my AR, which fights me to my HD Pump Shotgun.