x264 is better, both in terms of sound & Video, but the filesize can vary, whereas xvid is usually blockier, but the filesize is usually around 350mb for a 40 minute tv show, or 700mb for a 90 minute movie.
mp4 files can range from 260mb to 410mb for a 40 minute show, and depending on the screensize and quality you want, mkv files can range from 125mb to 1.5gb for a 40minute show.
mkv files are usually much better quality than mp4, an mp4 are better than xvid/avi
I prefer mkv, as the quality is usually good even on the smaller sized files, mp4 has been known to have out of sync problems regularly, but avi and xvid can usually be played on most dvd players with a card or usb slot.