Question about DSLR video?

I don't know much about DSLRs, or video and shooting it. I know that auto-focus is shunned upon when shooting photography, of course. But Do I want video autofocusing on a camera? how does that work? Is it lens based, or body based? I heard something about, with the live feed from the swivel screen, you can't have autofocus when that feature is turned on? Is that true for both d5100 and t3i? I am looking at buying t2i, t3i, or d5100. Or another DSLR if anyone can recommend one for video! Please help me separate pros and cons between them so I can buy one!