If the extra of 100mm reach is really the deal-breaker, I'd happliy go with the 50-500mm. It's a superb lens, and probably sharper than the 80-400mm,
if your hand-holding technique is good enough to get the best out of it.
But personally I've
never found myself wishing for an extra 100mm of reach (an extra
400, maybe..!
) and as an all-round bird photography lens the 80-400mm is really hard to beat.
As I suggest in my review, an HSM motor in the 80-400mm would be nice, and I'd be very surprised if the next version doesn't have it: but the internal AF motor on the Nikon mount version of the lens does a very good (if apparently slightly noisy
*) job, and little things like getting into the habit of pre-focussing at roughly the right focal length can make up for the lack of HSM.
In truth though, I don't think I've ever missed a shot purely because of slow focussing - I've missed a few through the lens
hunting because I was trying to shoot in a situation where the AF would struggle - that's more down to the camera (and me!) than it is the lens, and most lenses would hunt in those situations.
* I say that the lens is
apparently slightly noisy, because when you're taking a shot, your ears are only inches away from the lens.
If you hold the camera down, or at arm's length, and trigger the AF, the sound from the lens is really just a whisper, and there's no way it would disturb any bird you were photographing.