For a C300, maintenance is $875 a year for the three years of the lease (that’s what I was quoted). You can roll that up with your monthly payments. The factory warranty should be fine for anything else you might need during the 3 year lease, so I would only pay for that maintenance plan.
Residual value of a CLA is now commonly around 59%, while that of a C300 is 62%. Because of that, I would say the C300 is more amenable to good lease deals than the CLA.