The only difference between the two men is that Ted was elevated to sainthood by the liberals.
A difference between Kennedy and Stevens is that Kennedy was a tremendous influence in the Senate who produced many pieces of signature legislation from the 1960s to the 2000s. Many people disagree with that influence or think it was bad for the country, but for better or for worse, he was a major factor in what our government has become. Some people also think Chappaquiddick was more important than all of that, and that's a debate for another day.
Stevens, by comparison, didn't have many comparable legislative accomplishments. The best comparison for his accomplishments, to be generous, would be Sen. Byrd. Longevity and intense devotion to financial support of his home state.
Edit: px said it much more succintly (and less senatorially!) than I did.
