Purely in terms of sharpness, the f/4L IS is thee best one among Canon's L zooms, from corner to corner wide open. f/2.8L IS cannot really compare to it when stopped down to f4, only say "similar"


So if image sharpness and/or light weight and/or cheaper price are what you are looking for, f4L IS is definitely the one to go.


Otherwise, get the f/2.8L IS without a question.