You should use the search function.
http://teamfortress.tv/forum/thread/8384/1
Platinum and myself made some pretty good points in here (24 & 20, respectively), both of us having been main callers for our respective teams for quite a while.
To answer your question in the latter part of your post, probably demo/pocket, as they're typically leading the helm of a push & the highest damage dealers. Medic is also with these players the most, and is generally able to see the majority of the positioning and health of their team. In the end, it really just comes down to preference and style.
eg - I main call for immunity. On the flipside, knoxxx and numlocked maincall(ed) for epsilon.