If you are running short of time, select an optional paper which requires less time for a decent level of preparation.
If 1-2 years is available before the mains exam, a hardworking candidate might be able to finish standard textbooks of almost any optional subject.
Please keep in mind that all optional subjects are high scoring if you have knowledge in the subject and if you can answer the questions in an analytical frame of mind in line with the latest requirements of UPSC.