The truth is that it is a very bad time to ask the question.
Although I am, among other things, a Nikon user, I'd say that the most universal answer today is probably to get a Canon 5D.
I am saying that now is a bad timing because there are very insistent rumours that Nikon is about to annouce their own full frame (or nearly so) based camera. My recommendation would then instantly switch to Nikon.
Funny that I am working with a 5D and think that for outdoor use the Nikon D2x would be the better camera (weather sealed, faster, much better viewfinder, better display,..) General I find the picture quality of the 5D very good, but only with the best lenses (24-70 2,8 for example) and only if you dont get a lemon which happens in both camps frequently I fear. My new purchased 17-40 L never made it up to the optical performance of the 24-70 so I sold it. Waiting for that 16-35 II for 1700.-€, I am not sure if I will buy it for that price. (for super wide angle I do lot of stiching with really right stuff panorama plate, and that works very well).
If Nikon brings an almost full frame camera I predict that all Nikon users will start to find out the hard way, that Nikon is nothing better in wide angle performance than Canon. All optics of both companies were never designed for digital use, so unpleasant surprises will happen mainly below 28/24 mm optics. The main feature of a full frame sensor is ISO performance, which I needed when I purchased the 5D (weddings at candlelight for example - hard to do that with a Nikon). As I am now able to go back to more "outdoor" stuff again (never enjoyed the press/wedding stuff very much), I would say Nikon would suit me more. As mentioned ergonomics are important, I can work with Canon very well, but I had a D2X for testing a few days and liked it better (and actually I came from the Nikon D-100 camp, so I am used to Nikon too). Than there is the question of which optics you think you need, the more you go the wide angle the more you are better suited with Canon, but only with the expensive new 16-35 lens (look at threads about Canon users which adapt old olympus, leica or zeiss lenses to their cameras, they dont do it for fun, but because wide angle is a tough area with digital cameras). The more you go the Tele end, you will find advantages of the cropped sensor.
And also the Raw converter will have a very high impact on the quality of your files. DXO is amazing with landscape, architecture, but ACR is good with portrait, as it makes easier for a softer look (which is hard to get in DXO)
It is a hard decision, and the bad thing is, you might feel in a year that it was the wrong one, no matter what you did this time.
Good Luck !