I wouldn't discount the new Nikon D3X quite yet. Shouldn't we wait to see what it can actually do? IF they have limited the AA filter while keeping the ISO down a stop, that could be a very good thing ....then "Leadership" will be in the eye of the beholder.
I do not like Canon files because they are overly filtered & waxey to my eye. I vividly recall my heart sinking when I opened the first set of wedding images shot with the 1DMKII
The last two Canons I owned (1DMKIII & 1DsMKII) were the last straw, and I jumped ship when Nikon finally went FF.
I also seriously doubt that a Canon 5DMKII will be in the same class as the D3X any more than the 5D is in the same class as the D3. Personally I shot with the 5D for years and hated every minute of it. It's one of the few cameras I've used that I never took to ... no matter how much I wanted it to be so. Strictly a personal opinion.
However, IF I were still shooting Canon, I think I'd still pop for the 5DMKII because it's small ... and I'd use it with a few select ultra fast prime lenses ... however for versatile use, Canon zooms are in desperate need of a total overhaul as much as Nikon needs to address a few basic fast primes. The Canon 24-70/2.8 main-stay lens leaves a lot to be desired IMHO. And anything below 50mm prime or zoom range has some of the worst distortion I've ever experienced ... something I personally despise.
But a Canon 5DMKII with a 50/1.2L would be a pretty nice set up by itself. Wish the 50/1.2L had IS ... or the 5DMKII had internal IS. Then it'd be the perfect available light machine.