Still, the price difference between mounts is quite huge! For that kinda price I'd expect the latest OS version. I've never asked around in shops but is the price difference that great when bought new in shops as well? Or just on the resale market?
Do you shoot/plan to shoot a lot of sports? Since you have the 50-135 it seems like quite a lot of duplication eh, two large 2.8 tele zooms? Anyway I feel that 70-200 is better than 50-135 for sports as the DA* focus speed is slow being a micromotor rather than ring-type. Also, more reach is always better, especially for amateurs without sideline access at all events.