Should I do ops management from SP Jain or choose Great Lakes Chennai for its PGDM, where I get to choose my interested field (finance, marketing, etc.)?

When you follow your interests and passions, the result will be magical. This is the apt answer to your question. The primary importance should be given to your career and interests, not the institution. Of course, institutes play a major role in giving foundational support to excel in your career, but never waste more time in the selection process of the institution. Follow your mind, and if you are not sure about SP Jain, then opt for Great Lakes, as they ensure great academic support and the best infrastructure facilities.