Voodoo4u2, my friend, they are both wonderful lenses, no doubt, and I certainly can't argue that f2.8 is faster than f4. I just want to be that those considering the 2.8 understand that they may be tempted to leave it home because of its size and weight.
I'm kicking myself not taking my whole kit to a concert last night. I only took my 24-105 f4L IS because I'd be shooting the band, kids, sunset, etc. On my way back to my car I hear a bird of prey (I'm still trying to figure out what young bird this was) screeching repeatly. I found him on the peak of an apartment building and think, what the heck, I'll do my best with my what I've got, so I crank ISO to 3200, extend the lens out to 105mm and start taking the photo below. This is a 500% crop!!!! Of course, you see all the noise. It would have had some noise even if taken with my 400mm f5.6L, but the crop would have been like 25%, making the noise harder to see. I had the right tool, but it was sitting at home by the front door.
My whole kit is heavy, cramed in a full sized backpack, but I'm seriously thinking that I'll take the whole thing to any photo-worthy event, even if it's in the trunk of the car. My fastest lens if f4 and if everything were an equivalent 2.8, it wouldn't all fit in a backpack and I'd have to consider a rollerbag. So, I can't argue with more light, BUT you've got to carry all that stuff AND some of these smaller lenses are actually sharper (slightly
).
Taken at 1/1250, f4, ISO-3200, 100mm, cropped about 500% with added NR applied in DxO Optics Pro. (BTW, I added some EXIF to the bee photo, thanks).