hello all,after searching every site for a ef 400mm f2.8,and finding one,i have also found a ef 500mm f4,my main photographic topics are motorsport and rugby.now heres the question..........which one of the 2 lenses would you buy for what i do.taking into consideration the weight of both,aperture size and finally the most usable lens of the 2. i would like to hear your comments or experiences with either.cheers paul