[muydeemer]

It's an ill-defined question. The correct question should be 'How much can I learn/gain/develop from an extra master's degree". Degrees are just papers, they only matter during CV screening. What matters is your set of skills, abilities and expertise.

[Fej]

That was part of the intention of the second question. I don't have an answer for myself, unfortunately. I'm searching, of course.