Having your own private SSL is so much better than using a free shared SSL. The private SSL helps you get more credibility because you:
1. Avoid security warnings
The shared SSL generates security warnings every time you wish to access a secure page. To avoid such popups you need a private SSL.

2. Avoid host's name in your URLs
In order to access a page with your shared SSL, you need to put your host's name in your URL. To avoid that, you better get a private SSL.
With shared SSL, your secure url looks like: https://secure.yourhost.tld/~username |
With private SSL, your secure url looks like: https://secure.yourname.com |