This one is almost a college course.
I know all of the answers, but it would be a full page of technical explanations of what is what.
One thing I can tell you for sure; There is an absolute difference between a $40 compressor and a $1500 compressor.
The point of having a good compressor is to get the best quality files at its smallest size. If you get this formula right, you can have the right quality formula with a reduced bandwidth bill. This is so important if you have a large amounts of video, I can't even stress enough. And it will be h.264 wrapped up with MP4, .mov, etc.
And for 4K look out for h.265 wrapped up with MP4 coming out soon.
I learned this when I was shooting 3D content 4+ years ago. I was shooting 2 EX3s and overlaying both files over each other. (Massive Files)
The 3D pop and depth looked best uncompressed. As we would compress both over laid files, the pop and depth would noticeably get worse and worse as far as the 3D was concerned.
This was my 1st experiences with actually being able to measure different compressors and how their algorithm worked. This was done with a shit ton of reading with actually testing out what I would read. I was paid very well of my research and answers.
Thats when I saw for myself that cheaper compressors were in fact that..Cheap!
I would work on this 12 hours per day for months with visual tests after tests.
So without getting too into it, my solution for the best compressor out there is Sorenson Squeeze.
http://www.sorensonmedia.com
I use that and or Matrox with Max with Apple's compressor for quality and speed.
What ever you choose, you will still have to test, tweak and test and tweak until you get what works best for you.
Hope that helps a bit.
Just Dave