The major difference in between a shared & dedicated Secure Certificate is the appearence of URL and how it will appear to your customers.
With a Shared Secure Certificate, the URL may look something like this:
which means the name would be displayed as https://jaguar.dnsracks.com/~yourdomain/ Although this would be fully functional it may not look professional as you are sharing a certificate with someone else and may turn off your buyers
With your own (dedicated) Secure Certificate, the URL is determined by you, either as another registered domain or as a sub domain of your website domain name.Their's an associated cost of US$ 99 which includes the cost of 123 thawte SSL certificate + installation + dedicated IP cost which is required for the dedicated SSL to run.
Use of a dedicated Secure Certificate appears more professional to your website visitors, helps in "hiding" our identity and would give more confidence to your customers.