70-200 f/4 L USM is an excellent lens. Test charts are test charts, but in practice using my 60D I can't tell much difference between that lens used at f/4 or the 135L used at f/4.

Plus, it is relatively cheap! I picked up mine for $559 USD about a month ago.

I rented the 70-300L once and thought it was a great lens although a lot more expensive. It's very sharp, has a great range, and has a 4-stop stabilizer.

Dave