Primes are usually a tad sharper, the 100-400 is more flexible, has 2 stops IS, for birding longer is usually better, so also consider Sigma 150-500 (better IS system than the Canon, but a bit heavier, but also longer) and the Sigma 50-500. Or if money no object the Canon 500mm f4 is considered a good lens. Personally I would go with a zoom.