oh. I am interested to know why you choose canon at the end?
I am hesitant to reply as I do not want to start a Canon/Nikon debate. I shoot primarily sports and a bit of theater, obviously this includes a lot of indoor/low-light events. I do 4-5 events a week. I decided to keep the Canon MkIV and sell the D3.
Since I had top-notch glass for both systems I felt my comparisons were pretty much apples to apples. I am also a JPEG shooter.
Reasons - in no particular order.
1. Liked the ergonomics of Canon body better. Every time I would grab the D3 I would inadvertently spin the front control dial. Thank goodness for the "lock" function. The Canon grip fits my hand like a glove.
2. Color. I just like the Canon colors better. Nikon tended to do weird things to skin tones. I can look at images posted here and usually tell right away which are Nikon. They just have an odd pink cast to skin tones.
3. Speed of focus acquisition on the Canon is MUCH faster. This drove me crazy when trying to use the D3 for volleyball. I'm not talking about tracking (which the D3 does very well), I'm talking about speed of initial focus acquisition. In any low-light situation with the D3 it was press the shutter, count to three, CLICK. With the Canon it is was virtually instantaneous.
4. AWB. When shooting theater the Canon yeilded spot-on colors under a variety of lighting conditions. With any indoor lighting the D3 always tended to turn everything orange.
5. As a long time Canon shooter I simply have more "stuff" for Canon.
That was pretty much it.