Hello,

As far as eligibility criteria is concerned for UPSC and WBCS it remains the same that is passed bachelor's degree in any discipline from a recognized university, for general candidates the upper age limit is less in UPSC compared to WBCS, UPSC has quite has extensive syllabus and state level civil services in general do share a lot of commonality with UPSC, but yes unlike in UPSC you don't have to study in such a detailed manner in WBCS, if you're preparing for UPSC, automatically you're going to cover a large portion of topics, there might be few difference which you have to take care, this also requires a making a timetable and covering the topics that requires more attention, hence to answer your question you can prepare for both the most important thing is making a schedule and accordingly assign time to subjects for studying, you can go through the syllabus of both the exams available at the official website respectively, this will give you an detailed idea regarding both the exams, it's more dependent on your  preference, determination, focus and persistence as both these exams are highly competitive in their own way.