How does a security group sid differ from a per service Sid?
A SID is a mechanism that assigns privileges to the service itself, rather than to the account under which the service runs.

For Clustered installations:

It enables you to use a domain account with minimal privileges on the box, which also improves your security because people end up knowing the domain service
account credentials. 

For non-clustered installations:

Per-Service SID, it allows you to use Network Service as the service account, improving security by
getting automatic password management, but without the traditional downside of having Network Service accumulate excessive privileges from multiple services.

Refer to : 


SQL &amp; SIDs why we need it and what the hell is it?
SIDs and IDs