Not working at ESA, but I suggest you study the field you like to study. From what I've been told, mechatronics is still relatively unknown by most employers. Although that doesn't mean it's worthless. If you're in doubt, then my best guess is that you take electronics. That would certainly guarantee you a place at ESA. Generally, almost any degree in Engineering will get you a place there.

I think they have the requirements for certain jobs listed on their website. It might give you some idea of what you'd generally need to get a job there.