I also prefer the pump action over the autos. My preference in shotguns would be the 870 as well. I own one. The barrel can be changed out so you could have a smooth bore and a rifled barrel.
The Mossberg 500 is pretty nice as well. I have used both, I prefer the 870s action.
I've shot them too. I have nothing bad to say about them at all, but I still prefer the pump actions.
NEVER...NEVER underestimate the power of a good slug.
Amen. I've actually shot a car, 1970(ish) International truck to be specific, with a 12ga slug. It went completely through both doors. They're pretty powerful.
I just like the feel of pistol grips, especially since we'll almost certainly get a slightly shorter stock on the weapon for my wife
Has she fired one before? I'm not accusing you of not doing your homework, but if she hasn't, she might not share the same affinity for them that you will.
Larger shells were made for long range duck and goose hunters
Specifically to shoot steel shot because it's illegal (in every state as far as I know) to hunt water fowl with lead and steel loses velocity fast. Without a heavy load trying to shoot down a goose is like trying to shoot down a plane.
Personally I'd stick with the 3". The 3" will chamber a 2.75 or a 3 unless they've changed it. It's been a while since I've purchased anything for it, so it might have all changed by now.
I also agree with some of these guys suggesting a rifle. Depending on how you are going to use it, a carbine could be a better choice. 12ga ammo is bulky and heavier than a lot of rifle rounds. Depending on the gun, they can have a substantially less kick.