I just bought a 5D2. My primary requirements were good low light and some higher resolution for cropping and still printing large. D700 is the low light (and allegedly AF champ) but I just didn't like the way it felt in my hand despite the better build quality. In addition, I would be taking a resolution drop from the K20d, but the D700 is amazing at iso 3200 and 6400.
I actually preferred the SR on the Sony, and the low iso files on the Sony beat Canikon imho. BUT, the Sony really starts to struggle at 800 and above. Since I was wanting usable 3200 and 6400, that was a deal breaker, along with the fact that I just don't like Sony as a company.
Canon was the "compromise" camera. It is not as good at hi iso as the D700, but better than the others. The AF is less sophisticated than the D700, but I am used to paying attention to the AF with Pentax. Plus from what I read center point single shot on the Canon is quick and accurate. The resolution is great, and the video is a plus as I will likely end up using it.
But in the end it was the glass that made my decision. All of the f2.8 zooms are expensive and large/heavy from all three, though the 28-70 Canon is hundreds less. I love the Zeiss glass but don't like Sony and the hi iso results so then it became Canon vs. Nikon. Nikon has some great glass, but they have not updated many of their primes and they don't have good reasonably priced and relatively compact good zooms.
For me, the Canon 70-200/f4 IS was the tipping point. I loved my 50-135* Pentax, and that Canon is very similar. Also, the Canon 50/1.4 is under $400, well built, and has USM (silent AF). Plus Canon has the 35/1.4, 50/1.2 and 85/1.2 lenses. Yes, they are heavy and expensive, but I can see myself buying one or more of those over time.
So it really depends. I don't like the user interface on the Canon - I end up having to work around it instead of having it work with me. But you can usually learn to deal with those things. It did make me appreciate how good Pentax is wrt UI and usability. Plus after doing some pixel peeping, frankly going from a good APS-C (like the K20d) to the 5D2 isn't night/day difference. They render differently, and have differet DOF. The 5D2 does make very nice 3200 files, and 6400 is usable. The higher iso is just for emergencies, but nice to have it.
I still think that APS-C is a sweet spot for quality and size/portability. WHen you go FF. be prepared to spend a lot of money on good glass. Otherwise you'll actually be worse off. You really need fast/good glass to realize the advantages. Most are frankly better off with an APS-C body and some glass optimized for that (like the DA*s).
Shots from the 2nd day/night with the camera. Most cameras could get the first set, a couple of the second set are in near darkness. While the 50/1.4 did hunt on some of the shots, if there was anything to grab onto it would AF. And I like the look of the raw files at 3200.