I've had my 5D I for a few years now, and while it's served me well through many portrait and wedding shoots, I think it's time for an upgrade. I do a few weddings and paid gigs year, but photography for me is mostly a hobby and so I can't really justify putting out $3000+ for a 5D III.
The 3 things I really wish my 5D I had are 1) better AF 2) sensor cleaning and 3) AF microadjust. The lenses that I use most are all under 135mm in focal length: an ultrawide, my Sigma 50, and a Canon 100 macro. Built in flash commander wouldn't hurt, either, since I'm currently using a 580EX to commander flashes when I want to do E-TTL off camera flash.
The latest rumors (from a few days ago) have the Nikon D600 being announced on September 13th at a price of around $1500. I don't know when it will actually be available to order, but I presume by the end of the year. If I was to get the D600 I'd probably sell my Sigma and get the aspherical Nikon 50mm f/1.8G.
Now, 1500 is close to the close market price of a lightly used Canon 5D Mark II. The 5D2 has things 2 and 3 on my list, but the AF system is exactly the same as the 5D I's. The 5D2's larger size will fit my hands better, since the D600 is said to be similar in size to the D7000.
For someone in my situation, would it make sense to go to Nikon and get the D600, or would the 5D2 be better, and why?