Why woudl you want to do that NOW?
Canon's 24-70 II and 70-200 II L DESTROYED Nikon's counterparts, at least based on 5 professional reviews.
5d3 beats D800 and 1DX beats D4, based on 3 professional reviews!
Not a good idea now. Maybe next year.
Finally, even Nikon fascist Ken Rockwell has finally swtiched to Canon after 28 years of using it!
Easy answer in fact.
1) Better sensor (Sony EXMOR)
2) Better metering system
3) Better flash exposure
Out of those 3 points only point 1 really bothers me. As for Nikon: I don't love the company, nor do I hate it. I don't feel a connection with the brand, but that doesn't mean I wouldn't buy one. I appreciate their cameras and what they are achieving with them. As for Canon, I do feel a "connection" since I've had a lot of Canon devices in the past. As for their DSLR's... I like the looks, the ergonomics, the glass, but I don't like their sensors because they are lagging behind the competition by a big margin. I hope they will be back one day to crush the competition but I have my doubts right now. If they keep lagging behind a switch to e.g. Nikon might be inevitable...