From what I have read and heard the 55-250 is the way to go. First it has Image Stabilization, second it rated really high. Check out Fred Miranda (
http://www.fredmiranda.com/reviews/s...?cat=27&page=3) and read their lens review section. As matter of fact there is only 1 non "L" lens rated higher than it and 1 with the same rating. The 75-300 MKIII gets a 5.6 versus a 9.0 for the 55-250. The 55-250 has 37 reviews so take the score with a grain of salt but the 75-300 is in the bottom 10% of all the canon lenses reviewed. Since I agree with most of the other canon lens reviews on the site I have give this rating some credence.
John