Thing is, in the double degree, you have to complete a Maths Minor anyway. Plus, if you intend to do 3rd year physics/a physics major, then you would definitely want to major in maths. 3rd year physics is basically really complex maths (not complex numbers, though complex numbers are used). One of my mates doing Sci/Eng is in 4th year doing 3rd year physics subjects and most of his lectures are just deriving equations. Also if you are still kind of interested in computer science, you may want to do a computational mathematics major. Also I've looked at the subject guide and it's possible to actually complete 4 different maths majors with only overloading for 2 semesters.

To be more specifically in mechatronics engineering, my 4th year mate is using a million or so matrices in his thesis so Linear Algebra which you have to do anyway is rather useful. Plus because mechatronics is pretty much a combination of mechanical, electrical and computer systems engineering, completing Complex Analysis would be very very useful