There is practically no difference whatsoever. Considering that your question address academic differences, there certainly are not many. To start, all of the computer science classes are the same for both. Freshman students will be taking CS100/125 for both majors, and progress in the same pattern. The differences, while minimal, actually seem favorable for the Math & CS major: they do not have to take Chemistry or Physics classes. Additionally, all of the math classes stay the same and it is only until Junior year that it changes. This allows somewhat more course freedom, in my opinion at least. The downside could be the requirement for a language (at least for some), but high school credits can transfer over. Check out these flowcharts - they have everything you need:
Math & CS: https://cs.illinois.edu/sites/default/files/files/flowcharts/Math%20and%20CS%20requirements%20flowchart%202013-03-22.pdf
CS: https://cs.illinois.edu/sites/default/files/files/flowcharts/CS%20requirements%20flowchart%20GENERIC%202013-03-22.pdf
While I have answered your question, it is also necessary to understand that a degree from Math & CS is just as, if not more, practical than CS alone. This question regarding “employable-ness” has been reiterated over at /r/UIUC for quite a while now, and I believe I have a great way to explain it. The ‘engineering’ tag, as
Employer:
Sorry, we only accept Computer Science graduates, mathematics is just arbitrary and has nothing to do with the disciple anyways!
Of course, this is not the case. Jeff Erickson, a professor at UIUC in Computer Science, has said multiple times on /r/UIUC that every Math & CS student is able to attend the engineering job fairs/seasonal fairs. The only stark difference is that Math & CS students will not be able to share ALL of the same resources. However, they will have access to the labs in the Siebel Center (CS Building), so everything settles out.
All in all, either is a good option. Good luck!