Remington 870 or Mossberg 500
Both are well built guns and reasonably priced. I prefer the 870 because they have a steel receiver, and an aluminum trigger group. The mossberg has an aluminum receiver and a plastic trigger group, unless you buy the military madel 590, which has an aluminum trigger group.
1)magazine extensions easier on the Rem. mossberg is five rounds unless you replace the mag. tube, Rem is four rounds with easily added ie. screw on extensions for a variety of mag. lengths and capacity.
2) tacticle accesories. When a company comes out with any new gadget, the Rem. 870 is usually the first pump they make it for.
3)functioning in dirty conditions, Mossberg has the Rem. beat here. However I've shot whole hunting seasons in mud and cold with a Rem. 870 without any problem.
These are some of the reason that I prefer the Remington 870, but I do own a couple of Mossbergs and they work.
As I stated at the top either of these weapons will work, see if one of your friends will let you shoot his 870, or 500 and see which one you like better.