What is the difference between shared and private SSL?The shared SSL is installed globally on the server and all clients can use it. Its certificate is a self-signed certificate and configured to work with Siteground server's name. That's why it will always show a popup warning when used.
On the other hand, the Private SSL is issued especially for your domain by a trusted authority. Browsers can validate the private SSL and will show a secure icon. Siteground definitely recommends the Private SSL for commercial projects. For more information please check:
https://www.siteground.com/ssl_certificates.htm