If you're using them to say that you are finished talking about something, they are both the same. However, "that's it" is also often used to express frustration or exasperation about a situation that's making you angry. For example, if your neighbor is playing music very loudly, you might say in a very angry voice, "That's it!" before knocking on his door and asking him to turn the music lower. It basically means, in this case, "I've had all I can stand!"

Agreed with the above. "That's all" can also be used to express that there's nothing else to add. For example, if your friend asks you why you look under the weather or sick, you can say, "I'm just tired, that's all". This means I'm just tired - there is no other problem. Likewise, if your friend asks what you did last weekend, you could say, "I just stayed home and watched a few films, that's all. Nothing exciting" You can also use "That's is!" when you discover something, like the perfect pair of shoes at a shop, or the solution to a difficult math problem.

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