Hii!
If you focus on preparing for UPSC and commit to a fixed target, I would say, go for anthropology. This is because, when you go for UPSC mains, it will help you. The syllabus for anthropology is short as compared to sociology and this gives you an advantage of time while preparing for the exam.
Nevertheless, all the subjects in the UPSC syllabus are equally important. The basic eligibility criteria just states the requirement of graduation degree in any subject from a recognised University. So, you can choose any subject.
Hope this helps.
Good luck!