Some say you pinch your fingers when pumping, but I found the 870's pump to be outstanding, with how fast and low-strength it is. Also, theres nothing better then firing a 3.5 inch Magnum with fast reloads!
I am not a huge fan of the 870. It is a good gun but far from perfect. It is very slow to operate compared to a Benelli Nova or Winchester 1300/FN.
Many accessories are available for the 870 so if you are into the equivalent of putting 22" wheels on a 4 X 4 pick up, knock your self out.
With the Mossbergs I only have experience with the 500/Maverick 88. I am not at all fond of these. If the 590's are significantly different, I would give them a try.
The Winchester 1200/1300 series are the fastest actions around (IMHO). The bolt to barrel lock up is very solid. The receiver does not carry any stress during firing. If you need to fire with one hand (injured), the 1300 will eject the spent hull and a simple forward shake will load the next round. They are kind of like a semi-semi automatic.
Winchesters are insanely great, from knowledge and sadly not experience. Someone I know has a semi-auto Winchester shotgun of some sort, not sure which model, and it looks awesome, I think it had like a 5 or 6 shot capacity.
I am a fan of the 870 also, but I dont have near the practical shotty experience of Robo so I will defer to the expert. The 500 is OK, like the safety position on top of the reciever vs the 870 but the 870 is my fav nonetheless. Just a pref I guess. As far as 3.5 I stay with 2.75 low recoil buck for defense rounds and the low (managed) recoil slugs are still good to 100yds with practice. YMMV
"I'm your brother, just make it count."