Sociology of CSS is quite different from sociology of PMS. In PMS, Sociology is primarily based on Sociological Theories, even the general topics like religion, gender, mass media etc are to be answered with reference to theories. In CSS, sociological theories are not given that much importance.
Moreover, the paper 2 of PMS is based on research methodologies.
So, if you want to opt for Sociology in PMS, you would have to work really hard to expand your basic sociological knowledge to theoretical and research perspectives.
Social Work is comparatively easy to prepare.
As far as scoring trends are concerned, both are scoring if you prepare them really well. If you don't have much time for preparation you should go for social work as you may be able to score well (around 60) with minimal preparation (as you have background of sociology). But if you want to opt sociology, you will have to work really hard but you will surely get very good marks but with minimal prep Sociology would prove fatal for you.
I hope that explains your query.