Video Compression

Just to make it clear to the OP the bitrate is to do with the quality of the movement of a picture. That is, if you have a talking heads you can get away with a low bitrate but if you have a action clip or something with lots of movement on screen and have a low bitrate you WIL get the blocky look to it, irrespective of which codex you use.

So bare in mind the type of clip you have.