Hi,
I think you already have SRM in your environment with 5.0 , But with 5.1 , Would suggest you to go with multisite mode because as there will be SRM in your environment, And for SRM you required Linked Mode vCenter and for Linked Mode , SSO should be with multisite because While the SRM plugin uses the vSphere "thick client" which does not use SSO, But the backend communication of the VCs will require multisite SSO.
Reference: http://blogs.vmware.com/vsphere/2013/02/linked-mode-with-sso-for-srm.html