"That's it" is used when some something is coming to an end, like a job or a party. "Well, that's it. The house is clean, finally" "That's all" is used more often when there is something that can be counted. "Is there another issue to discuss?" "No, I think that's all." Note that you could have used "that's it" with the last one. It is more of a preference that a hard rule.