SSL certificates differ from each other in both technical and non-technical aspects.
Some certificates secure for example one domain, and others secure all subdomains of one main domain. There are certificates that support older browsers with strong encryption; some certificates are fully supported by mobile devices; etcetera.
Regarding non-technical aspects, certificates differ in warranty, delivery time or re-issue conditions. Also the way of validation of a request can differ: an Extended Validation (EV) certificate is superior in this, which shows up in the high number of visitors that trust the certificate.
All SSL providers allow you to request a domain with www or without www. Both versions will be included in the certificate in case of a single domain ssl.
In case you order a multidomain ssl, you need to include both version (www and non www) in the certificate order.
From the product overview in your ssl panel, you can view and compare the specifications of all certificates.