Importance of Mathematical Maturity for CS Grad School Admissions
I'd go with the graduate-level class.  But just taking one class is not likely to make much difference.

What you really need is for your recommendation letters to specifically address this point.  Nobody will believe your claim that your grades do not accurately reflect your true level of mathematical maturity (although you can certainly demonstrate your mathematical maturity in your research statement).  But people will generally believe your references if they make the same claim, especially if they back up their claim with specific and credible evidence of your mathematical maturity, and more generally of your potential for research excellence.

This will only happen if you develop a closer collaborate relationship with your professors.  Don't just sit in the back of the random-processes class and get an A.  Talk with the instructor; ask cogent questions; ask about research opportunities, or at least directions for further study.  Impress them.  Give them proof.