Hello,

Sociology involves the study of society and social behaviour of human while antropology on the other hand not only deals with social aspect of human but even their biological aspect.

Choosing sociology will help you a lot as an optional in UPSC because of the similarity in the sociology syllabus as well as general studies syllabus,this will also help in general studies paper four in case of ethics, this subject is comparatively easy to understand especially for humanities students .On the other hand, Anthropology has comparatively shorted syllabus, as this includes social as well as biological aspects og human, so science is involved here.

You should choose the subject based on your interest, when it comes to availability of study materials, both Anthropology as well as Sociology have been taken up by previous qualified candidates, so there's no dearth of study materials, nowdays preparation for UPSC isn't just limited to printed books, one needs to use the Internet extensively as well regarding this matter.