i seriously considered the Bigma, just for the extra 100mm of reach... on my 30D, that would translate to an 800mm equivalent lens. but the Bigma isn't stabilized, and the EX 80-400 is, so i opted for that one instead. it's only marginally smaller - 12" extended, or 16" with the hood, but i can hand-hold it at full zoom and get clear shots, thanks to the stabilization. and since shooting birds in flight from a tripod usually doesn't work well, i wanted something i could shoot by hand and get decent shots.
I was also considered between the bigma and 80-400VR and decided on the 80-400. I think be able to hand hold the camera is an advantage over 100 mm extra focal length. Chances for taking flying birds is dim with lens on tripod. I had more than 8,000 bird pictures with my Nikkor 80-400VR, thanks to my decision to buy the 80-44.