70-200's - F4L IS v F2.8L IS

Hi all,
I'm going to the USA in 4 weeks time and was looking at buying a 70-200 while I'm over there.
I've been thinking the F4L IS is the way to go because it's much lighter and smaller (I have a 350D).
The 2.8 is obviously a better lens, but at $1000 and quite a bit of weight more I am a little undecided.
Has anybody used both these lenses and have an opinion?
Thanks
David
I'm going to the USA in 4 weeks time and was looking at buying a 70-200 while I'm over there.
I've been thinking the F4L IS is the way to go because it's much lighter and smaller (I have a 350D).
The 2.8 is obviously a better lens, but at $1000 and quite a bit of weight more I am a little undecided.
Has anybody used both these lenses and have an opinion?
Thanks
David