I dare say they differ from one another. ''Or not'' is an expression added to the end of a sentence to convey the idea that there is a possibility that the answer may be 'no'; whereas ''or what'' is used when you want to ask whether there is another different reason, argument, or action concerning something. Moreover, the usage of ''or not'' is rather formal as opposed to that of ''or what''.
The idiom ''or what'' is used in a way of adding emphasis to a yes-or-no question the speaker has asked, in which ''yes'' is expected to be the correct answer (In effect, if it wasn't what I said, what is it?). To make it easier to understand, consider replacing it for ''or what else can it be?''
Here are some examples:
1) ''Look at what I am wearing! Is this a great jacket or what?''
2) ''Is he crazy, or what?''
3) ''Come on! Are we going or what?''
NOTICE: ''Or what'' is a phrase following a statement that adds emphasis or suggests an option. For example, in ''Is this a good movie or what?'' the phrase asks for confirmation or agreement. However, it also may ask for an alternative, as in ''Is this book a biography or what?''. In the 1700s it generally asked for a choice among a series of options, and it still has this function, as in ''What is she interested in? in sports? in poetry? in medicine? or what?''
Hope you have understood it!