I'll second the idea of carefully patterning your gun before making the final decision. I spent a couple hours and quite a pile of various shells patterning my 870 shortly after I got it in order to decide what I should stock up on and what to leave on the shelf. I was rather of surprised at the results. Prior to patterning, I had sort of decided on using 2.75" 00 buck magnum shells (12 pellets @ 1290 FPS) thinking the added pellets would give a better, more complete pattern and the slightly lower velocity would be a little better in terms of over-penetration. As it turned out, they didn't actually pattern all that well with my gun. The standard 2.75" 00 9 pellet loads patterned quite well, as did the 3" 00 15 pellet magnums. The 3 inchers are lower velocity yet at 1225 FPS, but they give a pretty nasty blast for indoor use. The 2.75" 27 pellet #4 buck shells actually gave the best pattern, being nice and round and extremely full and evenly distributed. As a result of the patterning, I decided to stock up on those three.
As a result, I keep the 870 loaded with two #4 buck shells first because I use this gun frequently to dispatch pesky critters around the farm and the fine-patterning #4 is great for that. I sure wouldn't want to be on the receiving end of any of that either. The next two shells are plain old 9 pellet 00 buck for two-legged critters that might cause trouble. When I get a dedicated HD gun, I'll probably keep it loaded with the 9 pellet 00 buck with some slugs and maybe a couple 3" 00 mags in a sleeve. Not sure yet, but my point is the patterning was very helpful to decide what will work best for my particular gun, and it's nice to be able to keep a simple stash of just a few varieties of ammo that you KNOW will do what you need it to do.
EDIT: I would seriously consider using #1 buck in place of the 00 if it patterned well and I could find it around here. Unfortunately, I've never been able to find so much as a box so I don't bother with it. Personally, I like to be able to just go to the store and pick up what I need when I need it, and 00 buck is everywhere.