I'mma is short for "I am going to..." when you're talking about an action you're about to do. So you could say "I'mma go to the bathroom" or "I'mma get some sleep," but you wouldn't use it if you were talking about going to a place. So for example you couldn't say "I'mma the store," you would have to say "I'm (I am) going to the store."
Ain't is a contraction of "am not"/"are not," but colloquially you can use it in place of pretty much any negative contraction. It can replace am not, is not, are not, did not, has not, have not, had not, etc.
"I'm not ready” → "I ain't ready"
"They aren't ready" → "They ain't ready"
"It isn't ready" → "It ain't ready"
"I didn't do my homework" → "I ain't do my homework"
"I haven't done my homework" → "I ain't done my homework"
... The list goes on
I would say that "I'mma" is used much more often. I'm a native English speaker and I almost always say "I'mma" or "I'm gonna" instead of "I'm going to."
"Ain't" is only used sparsely in the Southern US