I'm just going to toss a generic "don't choose your weapon by what the Navy SEAL SOCOM SWAT SpecOps operators use" admonition out, here. Use what feels and operates best to you - if shotguns have one common thread, it's that they're built to last. In all probability, it'll outlast you. Doesn't get much better from a value-per-dollar standpoint.
Filling the "tactical" niche are the Mossberg 590 and the 870 Police/870 Tactical. The 870 is the most popular pumpgun in the US, commonly used by police, and it used to be true - still might be - that more Remington 870s have been manufactured and sold than all other pumps combined. The 590 is the issue shotgun for the Army.
If you're looking to start with a plain-jane gun and add things as they suit you, the Mossy 500A/Maverick 88 and Rem 870 Wingmaster/Express are the "normal" versions of their respective lines.
The 500 has an aluminum receiver that (mechanically speaking) will wear out faster than the 870's steel receiver, but (practically speaking) either one will probably still be seeing service with your kids when you're ambling around in a walker drinking prune juice and trying to remember your own birthday. The Maverick 88 is the "barebones" version of the 500, though I couldn't tell you what the differences are. Mossberg 500 forends are not compatible with the Mav88 without the use of a (cheap) adaptor, as the Mav88 rail assembly differs from that of the 500.
The 870 Wingmaster will come with a blued finish, slightly more attention to the internals (someone correct me if I'm wrong) and a presentable wood stock, in comparison to the 870 Express, which comes with a satin-finished beechwood(?) stock and a "dry" parkerized finish that is somewhat susceptible to rust; a quick wipedown with some sort of RIG (rust-inhibiting grease) should rectify the problem.
Expect to pay a little more for a Remington than for a Mossberg, generally speaking. Mossberg 500s can be picked up at your local Wal*Mart with a vent-rib hunting barrel for around $160 NIB, or in HD configuration for about $250-280, according to budsgunshop.com. NIB 870 Expresses run about $260-320 depending on store, rebates and barrel configuration. You should be able to find at least a couple of each at your local pawnshop; if the weapon looks well cared for, it's probably a safe buy. Shotguns don't wear out - they just wear in.