Generally speaking, neither of the options will hinder your career prospects / internships / grad opportunities, unless you have a very specific career goal that required a very deep dive into either math or CS.
It all depends on your long-term career goals. What do you see yourself as in 10-20 years? Figure that out and work backwards to present time to understand your short-term plans for the next 2-3 years. Your diploma is not the goal, it's an instrument to get into the type of work you'll enjoy for years to come.
Definitely take an internship, regardless of if it's a math or CS related or which industry, etc. The experience will help you understand what work environment you may enjoy or what you may wish to avoid in the future. It will also allow you to connect the knowledge that you acquired at Uni to a practical application in professional world.
Also, remember that people start feeling more satisfied at work when they acquire certain level of skill for that function. With skill comes confidence, reputation, good pay, self-sufficiency and therefore job satisfaction. Not the other way around.
All the best,