I suggest you look at a 590A1 or 870 if you are competent with a pump gun. I would not suggest getting a pistol grip only. Whether you get a stock with a pistol grip, that is personal preference. If you want a stock with a pistol grip, I suggest going with the 870 due to the placement of the safety.
Since many situations that require a defensive gun occur in low light, I suggest getting a Surefire forestock with integral tactical light. Shooting a shotgun with a handheld light is no fun.
If you want to put a sidesaddle on the gun, just be careful not to deform the receiver. Also, I believe the Mossberg is a non-steel receiver so special care has to be taken with a sidesaddle.
The 590 and 870 are great tactical shotguns. If you're on a budget and can't afford the nicer tac shotguns, you could always get a Maverick (by Mossberg) 88 with a 18.5" barrel. Then just get a buttstock sidesaddle. It's a really affordable alternative, albeit not as heavy duty as the 590 or 870.
I had a Mossberg 500 with the 18.5" barrel and now have the 590A1. Both are good guns.
I got a deal on a Benelli Nova and used it as a camper gun when I was in bear country. It is lighter and runs like a champ.
Recently I bought an old Remington 870 with a full length barrel. Not really good for home defense but the thing that impressed me about this gun is that I took it out right after I bought it and shot a box of shells through it . It ran like a champ. When I got it home to clean it the internals of the gun were filthy and I was amazed at how the gun still ran.
Last year I got a deal on a used Benelli M1 Super 90. This gun rocks! It is a semi-auto and I have had one failure to eject out of about 350 rounds. The failure was due to a reloaded shell that warped when it was fired. The gun is a simple design, very well built, and is easy to clean. These are going to be more pricey than a pump.