There are two significant differences between using a private SSL certificate (your own) and a shared certificate provided by CFDynamics.
You are using someone else's certificate, and although it is just as secure (uses the same cipher strength) it may make customers uneasy during certain types of transactions, such as financial and highly sensitive personal information.
When using your own SSL certificate the URL will contain your full domain name. However, when using a shared certificate provided by CFDynamics the URL will contain the name CFDynamics and a portion of your web site name.
Keep in mind that while the security of the certificate is the same between shared and private, any insurance that you may receive with a private SSL will not apply to your site or transactions when using a shared certificate.