"I mean it" is more of an indicative statement. When I say the two phrases out loud to myself, I hear "I'm telling you" as flowing better with the following sentence. For example: I'm telling you, I don't know where the money is! "I"m telling you" seems less important than the following clause, "I don't know where the money is," which receives the force of the whole sentence. When I say, "I mean it, I don't know where the money is!" the force lands more on "I mean it" because "I don't know where the money is" becomes the object of what I mean. Actually, the whole second clause is only telling us what the "it" is from the first part. Where the first example only adds emphasis, the second example actually adds new meaning to the sentence. Instead of simply making a declaration, it turns into a declaration about a declaration. Does that help? Blessings

To me, they are reasonably different, or at least - you use them in different ways. 'I mean it' is usually used after a sentence where you say you will do something, and usually implies a threat. "If you don't clean your room, I will throw out your phone. I mean it!". To me - "I'm telling you" is more used to introduce an alternative way of saying something. "Those guys came out that door so fast. I'm telling you - they were a blur as they went past". Although - that phrase could be used slightly differently in other cultures.