my IQ doesn't seem to diminish when a new body comes along
We should all recite this mantra at least 10 times every day. My 1DmkII produces the same IQ as it did on the day it was made. It (and its cousin the mkIIN) was THE sports camera for over 3 years. It doesn't produce the detail of the modern super-high MP cameras. It doesn't have the high-ISO capabilities of modern cameras. But it isn't any worse than it has ever been.
As for the Nikon vs. Canon debate; get what works for you. I like Canon's lens lineup, so I shoot Canon. On the other hand, I'm in the market for a 5DmkII (to supplement my 1DmkII, not replace it), and while I wish it had the AF system in the D700, the D700 doesn't work with the 135 f/2L. But if I were forced to shoot Nikon for the rest of my life, I'd be just fine.
Get whatever works for you.