I think it's both difficult and unfair to compare Sivagami and Devasena. Both were strong women in their own right. Sivagami was a shrewd politician. She had the strength of speech, and she had a thorough knowledge of law and justice. In today's world, she would have made a fantastic lawyer.
On the other hand, Devasena was a warrior. She had the strength of arms, and although she too had a knowledge of law, she had her own intrinsic values which she cherished above law. In today's world, she would have made a fierce police officer, or some form of soldier who protected people with her own life.
One of them had the strength of education, of speech and her staunch belief in law. The other had the strength of character, values and physical prowess. It's difficult to compare them, as they both excelled in different areas, and they were both intrinsically good woman. Unfortunately, Sivagami's blind belief in the law she believed in, the law which was flawed, is what sent her on a downward spiral during which she lost her values and committed crimes that changed the entire fate of the kingdom.