Like Linux in general, there are few MythTV distributions you can get. Unlike Linux in general, most of them have specific purposes they work best for. The three popular ones I know of are:
MythDora is just Fedora with MythTV (and its dependencies). This is intended to leave you with a completely usable Linux installation that includes MythTV. It comes on DVD and is certainly the largest of the three.
KnoppMyth is either based on Knoppix or Debian (or really both) depending on how you look at it. Knoppix is a slimline distribution based on Debian and KnoppMyth was originally based on Knoppix. But I’m sure I read somewhere that the latest is version is just based on Debian but in the same way Knoppix is. Whatever the situation is, all you really need to know is that it is a minimal installation that leaves you with a fully functional MythTV installation but relatively little else.
MiniMyth is the smallest of the three and the most specialised. It only runs the frontend software and only the EPIA mini-ITX motherboards. Furthermore it is designed to run disklessly booting over a network - mainly as a silent set top box in your living room.
The only one I’ve actually tried is KnoppMyth which was easy enough to install. From what I’ve been reading they all seem easier than installing MythTV into an existing installation.