I vote for 400mm5.6, IS is not a big deal when shooting birds, birds almost never keep still even perching on the trees.you know the most fun part of birding is to capturing the action, that doesn't need IS, only fast shutter speed.
when the lighting is not bright enough to get faster speed, just stop shooting. all lens has it's limitation, so is the 100-400mm zoom.
i like the 400mm 5.6 because it's smaller, lighter, faster, better IQ and cheaper, also it can force you to use tripod when there is situation you need it, remember, this is your starting bird lens, later on you must need a tripod when you get your big white lenses, you should practise with the 400mm now.just my 2 cents.