A glance at Civil Services (Mains) Examination 2007 reveal that 96 candidates were recommend out of 1092 candidates appeared with Sociology as one optional Subject.
The Success Rate for Sociology was 8.8% in CSE 2007.
In CSE 2008, more than 1500 candidates appeared with Sociology as one of the optional Subject. Which clearly indicate increasing popularity for the subject.
Coming to your other subject, Mechanical Engineering which had success rate of only 2.6% as only 2 candidates were recommended out of 77 candidates appeared in Civil Services (Mains) Examination 2007.
Only 94 candidates appeared with Mechanical Engineering in Mains Examination CSE 2008.
Since you are to appear in 2011, you can revisit your plans for your optional subjects. Sociology seems to be a good choice and if you are looking for safe and better success rate, you can look for either Public Administration or Psychology.