1) The validity of Free SSL Certificate is varying from 30-90 days whereas Paid Certificate is valid for 1-2 years.
2) As compared with Paid SSL Certificate, there is no support, long time validity and warranty available with Free SSL Certificate.
3) Free SSL Certificate is the best option for blogs, personal websites etc. on the contrary, Paid Certificate should be installed on the heavy traffic e-commerce websites.
4) WildCard SSL is not yet available with the Fee SSL Certificate providers. It's still being implemented.