I used to have an 870 pump for sporting clays/skeet/trap. Loved it, it was my first shotgun, was extremely reliable, and it fit me pretty well (and I wish I kept it!). However, if you get serious and once you get better, especially on the sporting clays course, the time required to pump becomes a bit of a limiting factor. I've had the second clay get up behind the trees as I was pumping the next round on the more challenging sporting clays stations, and I wasn't slow on the first clay. I now have a Winchester SX3 auto and there's no problem with speed. That thing will shoot as fast as you can pull the trigger. Oh, and I like the 28 inch barrel for sporting.
And while I'm not too focused with home defense (I think any shotgun will do the trick), it seems like a pump might add one more activity to a stressful situation, and I could see how someone might just try to keep pulling the trigger and forgetting to pump the next shell. But autoloaders can jam if you get a weak shell or the gun isn't maintained. I guess it's personal preference in that instance. Also, while I can see and agree with Zimm's points above, for a novice like myself I think you would have to keep up with the handgun skills much more than for a shotgun (especially if startled, sleepy, etc.) for home defense purposes. Point and pull...let the lead shot do a good bit of the aiming for you.