Think about what you want to use it for. I recently bought my first, a Rem 870 Tactical (with ghost ring sights and that hideous "door breacher" choke). In my decision, I went to my LGS and tried out a bunch of the popular ones, being the 870s and the Mossy 500/590s etc. I do NOT like the location of the action lock on the Mossys, so that made up my mind in a heartbeat. I ended up with the 870 Tactical because I wanted the option of being able to swap out chokes without having to buy a brand new barrel (in the $100-150 range). The 870 Express (with 6rd tube) has a fixed cylinder and bead sights, retails in the $360 range. The 870 Express Tactical that I got was something around $490.

But my recommendation is to definitely check them out in person, because they handle and feel very different. The Mossy's also felt a little on the cheap side to me, too much rattle, but some people prefer them over the Rems, so its personal preference.