For productivity analysis, consider this simple C# LINQ query:

    var BestCandidates = DataBase.Applicants
            .Where(apl => apl.LookingForJob == "Y")
            .OrderBy(apl => apl.SkillLevel)
            .Take(10)
            .Select(apl => new { apl.FullName, apl.RequestedSalary });

How many lines of COBOL would it take to write the equivalent?
I think it is more than 100 (I know both languages).

You can do a fair comparison with some reporting and processing exercises.
Just make sure that the programmers know their languages well. For instance, a good COBOL programmer can defeat a bad C# programmer in some circumstances, especially in performant batch tasks (i.e. COBOL operates on flat files with direct memory access, therefore cannot equally compare to -let say- RDBMs with memory Objects, unless the programmer knows well about indexes, garbage-collector, direct file access, etc.).