The front side buss speed is 1066mhz so that the speed the board talks to everything including memory, it can't talk any faster so putting 1333mhz will run at 1066mhz. So I would go with 1066mhz and save your money. If you also make sure you firmware for the board is up-to-date and you are running latest OS it can take you should've able to run 16GB ram, even if apple say it only supports 8GB. If you get the correct ram you can run 16GB that's what I have in mine running high Sierra. In have same system as you. You may also want to think about adding a solid state drive in place of the drive that's in with will probably be a mechanical drive. Believe me you will seen the difference in speed when the system starts up etc.