Personally - I think I've seen this "heated" debate so many times, it's almost not posting again.
Buy BOTH - they are BOTH top notch guns. We are probably talking about the different between 9.3 vs 9.4 (in pump action) - meaning they are both A rate guns, and personal preference will tell the tale of which one you like better, as the differences are not very pronounced.
Out of all my research, this document helped me make my final decision:
http://members.tripod.com/~jth8260/870.html
For me primary concern was "ease of customizing" (without a dremel). Secondary was price. I went with the Mossberg 500 and have since added many many accessories with no problems what-so-ever.
And at $205 for gun + 2 barrels 18.5" CB, 28" IMP - I couldn't say no... (Big5 Sporting Goods).
6 months later... after camping trips, indoor and outdoor ranges - ~3 cases (700+ rounds of various #00 buck, #6 bird, 1oz slugs, 600grain magnum slugs), I'm still very happy with my purchase.
Also of note, the buddy I shoot with regularly has both (Mossberg 500 and Remington 870)... Personally I have not seen either of his pump action guns have any problems...
-M3