A shared ssl displays as https//ip/~yourusername/. It is less secure that private SSL because multiple accounts share the same secure connection. Buyers may also feel less secure about purchasing from your site if they are transferred from a URL such as http://checkout.mydiscountshop.com to https://142.94.0.12/~mydiscou/checkout.
A private SSL would display as https://www.yourdomain.com in the address bar. It is more secure as it is only used by your website.