If we handle the normal (full) postback and AJAX postbacks in a single controller action it is very likely that the method will be huge. A lot of if..else sentences handling the returns (returning regular views vs partial views vs JSON success strings), different handling for ModelState errors, etc.
However, splitting it up into two actions, for instance Create and CreateAjax means that two instances of similar code will have to be maintained.
What do you recommend to keep the actions simple but still cover both postback scenarios?
I would have two methods, Create and CreateAjax and look at refactoring the code so any duplicate code was put into a helper class or a service.