Hi Brian
I have a bigma which goes for walks with me almost every weekend attatched to my Canon 40d. It's heavy, but probably one of the best bang for buck lenses out there imo. The lack of image stablisation isn't really a problem because I rarely use it hand held, it's almost always attatched to a monopod which helps with carrying it around, over the shoulder. I get some funny looks sometimes I have to say
I hope some Nikon users can comment further on the Nikkor lens but having looked at some reviews about the 80-400 lens and the main gripe would seem to be it's fairly slow autofocus and not having full time manual focus without flicking a switch but it does seem to be sharp and comparitively light. Also has a poor minimum focusing distance but this may not bother you.
here's a link to one review I read but you should look for more-
Nikon 80-400mm VR
Usually when I use the bigma it is almost always at 400-500mm so I guess a prime would probably suit me better but I'd have to carry another lens, just in case.