In a fighting shotgun a semi will prove to be more reliable. In theory a pump should be, but human error will always be an issue and you will see far less malfunctions with a semi.
A pump is far less expensive. if money is a concern you get more gun for the money with pump. A pump would be my choice as a survival gun. Semi's work great with decent ammo, but pumps will still shoot crappy ammo a semi will choke on. They are also going to take abuse and work when filthy better than most semi's.
For SD purposes I'd take a quality semi. For a TEOTWAWKI scenario I'd take a quality pump.