Actually speaking. The comparision cannot be made on a general grounds. But, ya Reliance is considered to be the biggest group of India. They are giants, know how to grow...(by any means...)
Compared to that, Tata's you can not find any match in terms of corporate governance and ethics.They're the best. Tata is one of the groups, renowned for Socially accountable and the greatest contributor in Indian economy.
The comparision depends on the parameters on which you want to compare. If you want to compare in terms of assets valuations, may be Reliance shall stood No. 1...
Jyotindra..