IS is so useful and makes such a massive difference that I would not buy a lens in the 400mm class that didn't have it.
Unfortunately, this limits your choice severely: you can get the Canon 100-400, or else start thinking about the various mega-expensive Canon primes with IS - we are talking lenses in the US$6000 and up class here. Unless I've lost my memory (entirely likely!), none of the non-Canon longer lenses have IS.
Do you get better, sharper pictures with a prime? No doubt about it. Is it worth sacrificing IS to get that prime lens sharpness? Generally speaking, no. You throw away more than you gain. Nature photography is all about finding ways to cope with less light than you really want. You can never get too much light, and you are practically always looking for ways to get your shutter speeds up. With IS, you can get away with a lower speed if you have to (which is often). Without it, you are doing things the hard way.
(Can I have my 2c now?)
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