I know people from both JMC and Venky. Based on the things I heard from them, I will say Venky is definitely a better option for doing a B.Sc (Hons) in Mathematics. Here is why.

• Faculty - Venky has one of the best faculty bases in the whole university. The teachers are well-experienced and cooperative.
• Infrastructure - The infrastructure of the institute is great. There is a foyer, ICT lab, auditorium, and a well-equipped Maths department. 
• Library - The library has a great collection of books. 
• Fest - Exponents is the fest for the Maths department. You will get to meet other Math scholars in this fest. Other than that, there is the annual fest Nexus.

Considering all these factors, it is safe to say that Venky is definitely a better option.