Probably Canon for long, fast IS (Canon's name for what Nikon calls VR) primes - theirs are perhaps better, and certainly more available used and for rental. Nikon has the only pro-grade zoom in that focal range, though (the 200-400 f4 VR). Nikon also has an advantage in AF right now, as their top-tier AF is in every body from the D300 on up, while Canon uses compromised AF in everything except the EOS-1 series. One possibility to consider for birds is the D300, (great AF, cropped frame works great for telephotos, rugged body, comparatively cheap),which will actually give you more pixels in the APS-C area than anything except the 50D. If you need a lot of reach, and would be cropping if you had a full-frame camera anyway, the D300 is the best cropped-frame camera around.
-Dan