I did that in 2004 Feb from a 10D to a D2h and I regretted.
Nikon has got a great flash system (better than Canon's), great metering and the AF on the D2h was superb.
However, when it comes to IQ for pics > ISO 400. It just could not match up to the Canon CMOS.
The D2h sensor's ISO 200 was brilliant, produces very vibrant colours. However, beyond ISO 800, it loses detail, has too much noise . ISO 1600 was near unusable.
I moved back to Canon Dec 2004 and never looked back since.
Both can do a wonderful job. But frankly, from my own experience, Canon still has better overall image quality as I shoot a lot of Hi-ISO, available light stuff.
Canon works for me but not everyone has the same opinion. Since you asked, I gave my own experience.
Frankly, what matters is the $ you get from assignments and competitions that matter at the end of the day.