>How do you define a better programming language

A language that makes avoiding certain important classes of defects easier and more productive.

>how do you judge whether one programming language is better than another

Analytically, i.e. by explaining and proving how these classes of bugs can be avoided.

I don't find empirical studies on this subject particularly useful. There are too many moving parts in software projects. The quality of the team and its working environment probably dominates everything else. And these studies rarely take productivity and cost into consideration.