Well, I am guessing your talking about Nikon 70-200 or 80-400? Or same in Sigma flavor?
Anyway, the 70-200 will be the best if you want the best high quality lens. You will have the faster aperture f/2.8. And the 80-400 because of its large range will have a lower quality of output. But, we are talking in technacalities. As no doubt the 80-400 is a decent lens. Just with its large do all range, there will be some loss of quality. The 80-400 may suite your needs just fine as it has for many people. It really deppends if your looking for the best of the best. A a good quality lens.
Now something to consider. The Sigma 50-500 has a big following. Again its a jack of all lenses. But it has proven to be a good lens for not being from Nikon or Canon stables. Pricing wise you may be able to get the Sigma now if that range is what your looking for.
Ok, just went back and you were considering Canon 40D and 50D's. Canon has 4 70-200's. Need to know which your considereing. And their big range zoom is the 100-400? Or again the 70-200 and 80-400 in Sigma variety?