I would buy the 870, it is by far the most common pump with the most available parts and accessories out there. The only other pump I would prefer over an 870 is a Winchester Model 97 Trench. I like the exposed hammer for a HD gun and that fact you can trombone it is also nice. You can also slam fire the Winchester Model 12.
Gotta love the old Winchesters. Especially the 97's. I like the look on peoples faces when I would take out my Black Diamond 16 ga. 97
My house gun is a early 12 ga. 870 Wingmaster trap that I shortened the barrel on and did some other stuff to it as well. Remington and Mossberg both make great shotguns, but as others mentioned, there are a lot of aftermarket goodies for the 870 if you want to play around with it...