I'll step up for the Nikon user
I was a lifelong Canon man, with a bagful of A1's, motordrives, and more FD lenses than I could comfortably carry, but It was a real kick in the teeth when Canon changed the lens mount on all of the new bodies, such that the FD range became effectively redundant overnight.
So, when the time came to delve into the DSLR realm, I opted for Nikon, who seem to have had some thought for their long time customers, as many of their older lenses function perfectly well on their new DSLR bodies.
At the end of the day, there is nothing to choose between what comes out of the camera's of all of the main manufacturers, and as zan has said, once you are into a system, you are likely to stay with that manufacturer.
Canon however, might do well to take note of their past indiscretion - I know several other ex-Canon users who all changed brands as a result of the demise of the FD lens mount.
Regards
Mike.