I am a master's student in computer science. I have completed my coursework and will be graduating soon, with a project. I am planning to do a PhD in the future: is a master's thesis an important prerequisite for a PhD? Most people say yes, hence I am of two minds: (1) get an extension and improve my project into thesis, and then graduate or (2) graduate now, hoping that everything will work out fine. I already expressed my interest to work more on my project; my advisor said OK. Do I need to do anything other than this (improving my project, of course with research exploration) to get a thesis? My advisor seems to be a little uninterested, because he has to guide other students, too, rather than that I did a bad job on the project. But I am wondering which way to go: industry or master's thesis?
Your last question is a bit of a non sequitur, frankly: I don't really see the connection between going into industry versus doing a master's thesis with the rest of the question.
That said, JeffE's comment above is essentially correct: if you are interested in doing a PhD in the near future, an industrial job is probably not going to get you the research experience you need to be a competitive PhD applicant.
With respect to what you need to do to convert your project into a master's thesis, that's really something you ought to discuss with your project supervisor. Without knowing the specific policies of your department, as well as the specifics of what you did in the project, it's difficult (if not impossible) for us to know what needs to be done.