Is it better to do a master's degree in Management or in Marketing?

It depends, do what you like most :) 

I cant understand your question, management and marketing both are not one and same? because management is whole aspect and marketing is branch of it so various specialization is available in management i.e.marketing, finance, HR etc In my college or university such kind of specialization was not available at least so I don't know in other institute or university it may be. But important thing is what you want to do?

Both Degrees are Excellent, the choice depends on your Interest !!

i see significant value in both fields. therefore, my course was MSc Management with Marketing. its core was more focused on marketing, yet always analysed from a managing point of view. i would advise you to do both as a combo.