My vote is the Mossberg 590. Reason? High capacity, great ergonomics, good quality and relatively inexpensive.
Of course, you are dealing with a smoothbore here, so if you are wanting range, this wont do. But, for HD (buckshot or whatever) and mid-range (using rifled slugs), this gun would do just fine.
I would recommend Benelli, but as nice as they are, they are not the most practical of the bunch. And the Remington 870 (at least the newer ones...not including the Police model)? Well...no thanks.
Oh..and in terms of gauge, Id stick with 12. Sure, the 20 may shoot "softer," but given that there are plenty of "easy" firing loads available for the 12 (and lets not forget the 12 gauge is quite versatile as well), I personally cannot justify going with less...particularly given your mentioned size. Heck, Im 5' 8" @175lbs. and can handle a 12, regardless of load. Keep in mind, Ive only been "introduced" to this weapon within the last 15 months or so.
So, again, I would skip the 20 and go with something which allows for more "mileage."