Im looking into replacing my 55-200 lens and found the 70-210 f/4. I wanted something a little faster but the 80-200 f/2.8 is out of my range. So do you guys think that the 70-210 is better than the 55-200? I've never used one so I was wondering how the AF compares. I would need it to be just fast enough for sports. Or would you recommend saving for a while to get a 80-200 f/2.8?