H.264 at 12 MBit/s
should deliver excellent quality for HD content. H.264 at 24 MBit/s usually is overkill, even for 1080p material...
Just look at this 8 MBit/s 1080p H.264 sample:
http://files.x264.nl/force.php?file=...0.48.8mbit.mkv
(Looks excellent - and it was encoded with a pretty old version of x264)