We're grappling with this too. We are planning to install SRM 5.1.1 at our recently upgraded vCenter sites. Those vCenters are already in linked mode along with 3 that won't be SRM sites. All are using a single SSO server (not a basic mode install so we have flexibility to add others going forward) since it's in an ESXi cluster and since we have relatively fast WAN links.
There's a blog out by Ken Werneburg that might answer some of your questions but in short he says multisite is the way to go for linked mode SRM sites, because he claims that the backend VC communication of SRM sites absolutely requires multisite SSO but I've asked our account's engineer to look into this further because before I go modifying our SSO environment any by throwing a second one into the mix in our 'DR' site for that vCenter to use and subsequently alter our otherwise nice and tidy linked vCenters (in both the thick and web client worlds), I want to be absolutely sure this is in fact 'required'.
Here's Ken's blog (from February---so pretty recent) --- http://blogs.vmware.com/vsphere/2013/02/linked-mode-with-sso-for-srm.html