Well, the light on the shots you posted is quite different, the latter being warmer and generally better. That's also something to consider.
I did shoot for some time with the 70-200 F4L and have great memories of it. Pin sharp, light and really fast autofocus. I think it outperforms the 70-300 any day. Can't comment on the 760D as I haven't used, but if other newer Canon models are anything to go by, it should be OK, and a step up over the 550D.
Keep in mind that a change in equipment will always bring a learning period associated. If you elaborate a bit more why are you seeing your acceptance ratio drop we may be able to help a bit further
Cheers,
M.