Not is a more proper way of saying it. Ain't is not a proper way to use within a sentence. There is no difference, both means the same thing.

Ain't is a contraction for *am not, is not, are not, has not, and have not * in the common English language vernacular. ——https://en.wikipedia.org/wiki/Ain%27t

In English, "I'm not" and "I ain't" mean the same. The difference is that "I'm not" is grammatically correct English and "I ain't" is slang. Accomplished English speakers, when they want to adopt a somewhat defiant tone, or in some cases a comic or ironic tone, can use "ain't" to good effect. As someone who is learning English, I would suggest that you avoid it. As for "gonna", and its sibling "wanna", just don't. Ever. While these have become common in internet chat, especially among teenagers, and apparently on this forum, it is hard to imagine a use of the English language that sounds whinier, lazier and uglier.

I would suggest to refrain from using the word "ain't" when talking to someone as it would seem awkward.

Both mean the same; however, I would not use ain't unless I was just being extremely informal and wanted to appear uneducated. I might informally say, "I ain't gonna work today." But most know if I speak that way, I am trying to appear uneducated. Additionally, "gonna" really is not a word even though many use it. What they mean to say is "going to." So if you are trying to use common speech that is more educated, I would never use the words ain't or gonna in a sentence. At least not here in the U.S. Most do not speak that way.