I used a Tamron 70 to 300mm zoom on my Nikon D3000. I was zoomed all the way out to 300mm which on my crop sensor is the equivalent of 450mm on a full size sensor.
The other details include f/10 at 1/640th of a second and an ISO of 400. I was shooting handheld, hence the faster shutter speed. I manually focused as well. Auto focus is fine for some things, but for birds, I've found the only way to do it right is to use manual.
I'm thinking about entering this one in the phone company's photo contest. I was able to capture the meadowlark perched on one of their fiber optic markers.
Either one is going to be only as good as the operator. If you know a Nikon or Canon user well enough to share lenses, that's what i would choose. I'm partial to Nikons myself.