Mansit, It depends on the differences between the MS and the PhD. PhD programs are always research programs, so your statement of purpose is going to testify to your preparation for undertaking scholarship, your research interests, and your fit with the program in a research context. You'll name specific professors whose research interests align with yours; you might list some resources (like labs, archives, libraries, etc.) that the university has that make it a good fit for your work. However, if you are applying for a professional MS program, your statement will be a bit different. You still want to talk about experiences you've had that have prepared you for graduate study, but in addition to talking about research and some coursework, you also may want to mention professional non-academic prep (like work experience, internships, maybe even extracurriculars if they are highly relevant).