Choosing the Right Program
Math/CS technically encompasses 4 degrees: Math and Computer Science, Honours Math and Computer Science, Statistics and Computer Science, and Honours Statistics and Computer Science. There's a lot of overlap between the courses, and first year classes for the four degrees are almost the exact same so if you're on the fence you can try taking the honours versions of the few courses where the programs diverge and see how you find it. Honours programs tend to be more demanding and theoretical. They're intended to prepare the student for graduate school, and honours courses generally require more time than their majors equivalents. Although honours students get priority in signing up for honours courses, students in majors programs can also register for honours courses (these classes are rarely full, so you'll most likely never need to fight for a seat in an honours math course).
No matter what program you choose within the joint math and computer science family, COMP 250, COMP 273, COMP 206, and MATH 235 will all be required. In first semester, the only courses where you'll need to choose between the honours and majors version will be analysis (and even then, the math department lets students who took majors analysis 1 enroll in honours analysis 2) and possibly advanced calculus or probability if you choose to enrol in them in U1. If you're on the fence between enroling in math/cs and pure cs, it's worth noting that MATH 235 or MATH 242 can be substituted for MATH 240 to satisfy prerequisites. If you're ever in doubt about course selection, ask a departmental advisor (this is a different person from the faculty advisor that U0s are assigned to, and they will know a lot more specific information about your program).
The difference between a stats/cs degree and a math/cs degree is fairly small, especially in early years. The two programs diverge in upper level courses, with math/cs students getting slightly more freedom in their math course selections and stats/cs students restricted mainly to statistics-related courses. A math/cs student is required to take higher-level analysis and algebra courses but has some open credits and can use their math complementaries to take statistics courses, but a statistics/cs student will have a hard time replacing stats complementary courses with, say, topology. Taking probability early in your degree will give you an idea of whether you want to go the stats route and also opens the door to taking upper-level machine learning/artificial intelligence/natural language processing courses.
Why Math and CS?
Lots of reasons! A degree in math and computer science can come in handy in a huge variety of industries. Graduates have gone on to work in both industry and academia, finding software engineering and research positions at companies like Google, Facebook, and Microsoft, as well as faculty positions at universities.
Surviving Math and CS
The advantage of doing a joint program is that there are lots of different resources available from both departments! Burnside 911 is a great resource for math course help, Trottier 3rd floor is home to the CS Help Desk for your computer science courses, and both the Mathematics and Statistics and Computer Science departments offer a wide variety of cool talks and colloquia, including the following:
CS department colloquia: check the Computer Science Undergraduate listserv for a heads up on when the next talks are in SOCS.