I own and use all of these solutions.
The 1.4x converted is okay on the 70-200/2.8. I carry it in my bag for when I need a little extra reach. It does cut some sharpness and some focusing speed, but it works okay for many applications (journalism, etc.)
The 300/4 is a sweet lens. Very sharp, light and compact (for a 300). On a crop body like the xxD series it provides a lot of reach, something equivalent to a 500/4 with image stabilization. That said, I don't carry it that much. Most of the time when I want the extra reach of a 300, I also need the extra stop of the 2.8 lens.
I have the older non-IS 300/2.8. The AF is astonishingly fast. This is the lens I use when I need some reach, either with a 1D body, or with a 40D when I need just a little more. It's great for sports, news, pretty much anything I would cover on a daily basis. The downside is size and weight, of course.
If you really want to shoot sports professionally, the 300/2.8 isn't quite enough -- the 400/2.8 is more useful, IMHO.