My 2x11 cents:
1.Canon 5D3 will surely give you better low light performance. Please refer to the gazillion sites who've mentioned/proven this. This is a HUGE plus for HDSLR Video folks. Proof can be found in the 12800 ISO samples easily available by googling the same
2.If you look back to the earlier slr cameras by Canon (I believe I had the ELAN 50 something), some of the AF tech used here is the same and it never appeared in later cameras (NO idea why!). Having personally used it, I feel it's a huge plus. I used one of those film cameras back when I was in 9th standard (Yes:P) and had plenty of my "work" published/displayed. It was just so easy to focus that a kid could do it. (Yeah, agreed, that's not the point)
3.Megapixels never really matter to most users. In my 7 years of work, I've never really needed more than 10. Some of the best pictures I've clicked have been with my 1000D (10.1MP)
4.Weatherproofing: 5D3 has "improved" weather proofing but that doesn't mean it's guaranteed water proof. You're still gonna think a million times before taking it out in the rain. Also, the weather proofing really depends on whether you have a weather proof lens or not too. So please, purchase/drench carefully.
5.Pricing is on the higher side for the 5D3, the D800 looks more competitive. If you're primarily doing stills, the Nikon E version will probably render better results that the 5D3. If you're doing video, the 5D3 gains experience and features from the 1Dx and C300 lines and the ISO performance is unmatched. Check out some samples and you'll see what I mean. (IMHO)
6.Yes, 9 points vs 61 points will make a difference. But if you're shooting in good light, that shouldn't matter too much. New is always going to be better, but is it better enough for another Rs.60k?? (Have been shooting with a 5D2 for a while now: glorious!) Still, it WILL make a huge difference to people shooting primarily in low light
7.If all you need to do is click pretty pictures and share them asap on facebook, get an iPhone 4s. It's got an amazing camera and holds up in the toughest situations. Apps give you enough editing options AND it fits into your pocket. I have used one for about a week and it's gonna look as good as a Leica M9 on facebook if shot well, not kidding.
8.Higher fps: Matters for action photographers like myself. 4-6fps is huge if you shoot stuff that moves quick
9.If you have a warehouse full of nikon glass, please do not upgrade. It would be utterly pointless. Get the D800E, it will do the job just fine.
10.If you have a warehouse full of canon glass, please do not upgrade. It would be utterly pointless since both you and I know you're not going to be printing billboards (those things are 80% photoshop anyway, and the 5D2 will do it as well, if that's your hook).
11.If you're caught in a dilemma, be faithful. Stick to your camera. Go out shooting more often. Read more books. Imagine freely. Try to shoot things with your camera that you never thought you could shoot before. Explore it, inside out. Push it to it's limits. And if you reach a point when you can say that the camera can't replicate what you can imagine, UPGRADE.
Good luck to all buyers/dreamers/photographers