[You'd think that in a gun forum like this, some Senior Member might address the issue of the CROSSBOLT SAFETY versus TANG SAFETY with some clear comments, rather than all this wishy-washy "mellow-speak" but I guess political correctness counts for something!"
I'm not a "senior" member, but having spent plenty of time with both guns I'll address it. The tang safety on the 500 is slightly easier to use than the crossbolt safety on the 870. But the safety is of only marginal importance on a shotgun; personally, I never use them. The most important control on the gun is the slide release. I have always found the slide release on the 500 to be in an extremely awkward location. The 870's slide release is much faster and easier to manipulate.
The other thing to remember about the safeties is that the one on the 870 tends to stay attached to the gun, while the one on the 500 has a distressing habit of flying off. Replacing the safety button or some of its parts (like the little tiny detent ball) is probably the most common repair on a 500. It's only held in place by a screw, and when that screw works loose you can kiss the safety button goodbye. It takes five parts to replace that safety if everything comes off the gun (actually, the detent spring will most likely stay put, so you probably won't have to buy more than four).
If you've got a 500, be sure to tighten that screw periodically.
As for dual extractors, it's a good thing it's got two of them. That way, when one breaks (as they are wont to do), you've still got one good one. The problem is, the design doesn't work that well with only one extractor. In my experience the 870, with only one extractor (like the 1911, Mauser 98, etc., etc.) suffers far fewer extraction problems than the 500 does. The one extractor on the 870 is just designed better than the two on the 500.
If you've got a 500, keep the chamber nice and clean. They can be prone to pitting, and if that happens keep a cleaning rod handy to punch out the empty shells that won't extract. You'll also want to stay away from the Winchester bulk packs from Wal-Mart, because they tend to stick in the chamber and the 500 will really struggle with them (they're sticky in 870s, too, but I've never had to punch one out of an 870). The Remington and Federal bulk packs are OK.