This seemed to affect the US armed forces which were issued the Mossberg shotgun as well. To that end, Mossberg now offers the 500 and 590 Military models. This will cost you a bit more money, if you feel that having a metal triggerguard and safety button is necessary. The plastic parts on my 590 haven't failed me yet. The receiver of the Remington 870 is steel, while the receivers on the Mossberg 500s are aluminum alloy.
Steel is certainly stronger than aluminum, but at the cost of added weight.
Durability of the receiver has never been a problem for me, but take this into consideration. Surefire Responders and the Mossberg 590. Unfortunately, for the standard Responder you will have to grind down your bayonet lug. That's not something that anyone can do in their garage. Surefire has introduced the Responder model 623A1 which does not require removal of the bayonet lug.