If I'm editing a project, (prores422/PAL) what is the best compression rate to use to ensure the highest quality?

I've so far been using 'Export to Quicktime conversion' 1280 x 720 which works fine with Vimeo but somebody recently mentioned 1280 x 720 16:9 and I can't see the difference, even the file size is the same.

My current project is a non commercial one which I will put on YouTube and facebook as well as Vimeo, so should I stick to what I know or am I missing something.

The attachment in your post is from the Quicktime Export settings for "Size". That is size wrt the frame size not the file size. For your project, the two items won't make any difference. It's 1280x720 square pixels.

What does matter is the Video button on that Quicktime Export window. That is where you set the CODEC and the data rate (not compression rate). For 1280x720 output using the h.264 CODEC, high quality data rate is around 5000K bitspersecond (Kbps). Quick and dirty would be 2500Kbps. Either way, all you are doing is creating a smaller file to ease upload time to YouTube or Vimeo. Both services are going to re-compress the file for delivery at different playback rates (240p, 480p, 720p, etc). You may see the difference between a 2500Kbps file versus 5000Kbps but probably not the difference between 5000Kbps and the gargantuan original quality prores file.

I'm not sure about describing the technical difference but I can tell you about my experience with both.

Referring to your attachment, I think the top half is for computer resolutions and the bottom half is for TV resolutions.

I've discovered that if your project is HDV1280x720 and you export to "HD 1280x720 16:9" you'll either end up with cropping of the sides or pillar boxed top and bottom. When I've made this mistake I notice that if you open the exported video with Quicktime and you look at the info in the Inspector (menu: Window/Show Movie Inspector) that it will report a different "Actual Size" than 1280x720.

To fix/avoid this issue when exporting for YouTube, Vimeo, etc, always use:
"1280x720 HD" from the top half (of your attachment)
not "HD 1280x720 HD 16:9" from the bottom half.

Maybe someone more knowledgeable will explain this more accurately.
All my work is for the web too, but I'm guessing if you wanted to display your 1280x720 project on a widescreen TV you'd use the 16:9 export option.

The two sets of settings are for dealing with anamorphic video. HDV is anamorphic. Pixels in Anamorphic video are rectangular (long in the vertical dimension) and anamorphic video must be stretched out in order for things to be the right shape. That happens at playback time.

Thankyou both for the clarification. As all my videos thus far are for web use I have been doing the right thing all along but just wanted to be sure.