Which is more challenging between mechanical vs software engineering ?

I'd say mechanical is a much tougher branch considering the extreme diversity of courses it has under one branch and the relative depth into which it goes in comparison to software engineering. Software engineering is again a derivative branch of CS. Since CS is so vast, software engineering often gets overshadowed. A comparison between software and mechanical is unfair as the latter is a mainstream branch while the former is a derivative branch which is viable due to current market trends.