There is a difference, IIRC between these two:
RapidSSL certificates can secure both the bare domain and its www subdomain only *when* activated for www version. So if you wish domain.com and www.domain.com
to be secured with the same RapidSSL, you will need to activate it for www.domain.com
As for PositiveSSL (and all Comodo certificates except multi-domain ones) -- it is always issued for both domain.com and www.domain.com
, no matter which one is specified as the common name.
However, if you choose to secure a subdomain only:
For non-Comodo certificates, this feature works only when the certificate is activated for domain name -- not a subdomain.
PositiveSSL will secure both www.sub.domain.com
I'm describing only regular (not wildcard) SSL's above.
As for the pricing difference, it's probably the brand name.