The 100-400 weighs 1.38kg, the 400 1.25kg. Not a huge difference, but I think you'd find it noticable, and though I've never used one I've seen far too many great shots taken with this lens to think it's anything less than excellent. No IS is the big drawback, but if you can live with that it would make a good choice.
I can't imagine birding with anything less than a 400mm lens. Even 400 isn't really enough a lot of the time, but ignoring the vastly higher cost for the moment, a 500 weighs more than twice as much, and is decidedly difficult to handle.
I'm not a big man by any means, but I don't find the 100-400 difficult to lug around. You do get used to heavy gear after a while. Already, I'm starting to feel more at home with the monster 500 (is it habituation? Or actual muscle growth? A little of both I think), and now the 100-400 feels like a little toy lens. (Which is great!) My point here is that I think you would soon grow used to either of the 400s.
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