Normally one should select optional, with which one is familiar with, or has at least studied till the graduate level. If you are not comfortable with your subject of study at the degree level, then you should select the subject as per your interests after going through the available study material on it.
Even science and engineering students can take up subjects like history, sociology, anthropology, geography, political science, psychology and public administration. This is because there is a huge amount of study material available in these subjects.
Also keep in mind that you may have been proficient in the technical subject during your studies days but if you are out of touch with it, may make it tougher for preparing it all over again. In such case the fresher's may have an advantage and therefore any such decision has to take such aspects into consideration.
Remember the competition is among the people who have opted for the same subject. So try not to choose a subject that has too many people opting for it. Even though the focus has shifted to the General Studies papers, you have to do well in the optional subject to succeed in the examination.
Analyse the syllabus of previous years and the question papers, and analyse past trends. Get some feedback/advice from seniors and fellow students who are well versed in the subject.
Remember no subject is bad. History is good if you can spend more than 4-5 hours every day. Geography is a good option if you can spend at least 4 hours every day. Public Administration requires more than 3 hours. Sociology, more than 2 hours and so on....
So, it all depends on how much time you can spend a day and your liking of the subject. You have a very good memory then a technical subject may help else a general subject would be better. The Point is, if you are an electrical engineer with 2 years of experience go for a general subject like Public Administration or Sociology or History or Political Science. Take very calculated decision after going through all its pro and cons.