Disruptron takes personal skill and has a lower dps requirement, Lei Shen takes coordination, teamwork, focus over a much longer period, and has a higher dps check.
Of course you can compare difficulty - number of failed attempts before success. In which case, Lei Shen wins easily. Completing both encounters in the appropriate gear, Disruptron took me like 20-30 attempts as a feral back in 5.1, and Lei Shen took 250+ pulls.