With a goal of keeping size to a minimum, quality to the maximum while retaining the alpha channel (green screen shoot) what would you render to? Editor needs footage with green screen and edits in FCP.
I suggest Avid DNxHD 2.3.4 (free).
It compares favorably with ProRes 422 and will render the alpha layer as either compressed or uncompressed. Two-way compatible with Mac.
NOTE
when u go to Avid download site, make sure u select the correct links... Avid put the older versions ABOVE the current one... select the zip file UNDER the word "Downloads"... and don't forget to also grab the Mac version to give your FCP editor, in case he/she doesn't have it already installed on their box.