What YoYoMa said. I get Dell corporate discounts and the 690's that I bought were notably cheaper (but not a bargain level) than comparable Mac Pros. I ought to say price wasn't the reason for the purchase, backup from the manufacturer was. And the 490/690 are not crappy machines by any stretch of the imagination. The Precisions are not consumer machines and it is quite likely that if you are considering them you will come under one or more Dell discount umbrellas in any case.
Looking inside both machines (bear in mind though I've only seen pictures of the Mac Pro), the Dell is more conservatively engineered, while the Apple is more cleverly engineered with specific emphasis on low noise. The 690 isn't actually that noisy either, and is well under many 'homebuilt' PC's - so I'm quite happy. The advantage of the Apple approach is even lower noise. The disadvantage is - do you think they've learned their lessons this time? - increased heat.
With Dell, I also get significantly better support with a next-day engineer visit. With Apple these days it seems to be not so much a case of if it goes wrong, but when it goes wrong... so that level of support would be useful but alas, they don't offer it.
The Dells have not been trouble-free at first, but to Dell's credit they have made an effort to make things right now. Apple, when I had similar problems, did nothing of the kind. Ideally, I think I would like to have OS X running on the Precision 690. That would give me the best of both worlds IMO... but of course, this is unlikely to happen.