Hi,
Replication is a good option if you are following recommendations.
Symantec recommends to have 1 Primary & 4 secondary sites i.e 1:4
If you replicated 10 sites with Primary SEPM, your SEPM database size will increase depending upon number of clients across the sites.
Check following URL for more details.
https://www-secure.symantec.com/connect/articles/replication-and-considerations
I think GUP should be the first choice & if you wish to have redundancy, you can install an additional SEPM on the primary site itself and replicate with existing SEPM.
Group Update Provider: Sizing and Scaling Guidelines
http://www.symantec.com/business/support/index?page=content&id=TECH95353
Best Practices with Symantec Endpoint Protection (SEP) Group Update Providers (GUP)
http://www.symantec.com/business/support/index?page=content&id=TECH93813
New features and functionality in Symantec Endpoint Protection Release Update 5 (SEP RU 5) Group Update Provider (GUP)
http://www.symantec.com/business/support/index?page=content&id=TECH96417&locale=en_US
Video’s created on Group Update Provider on the Symantec Connect website.
https://www-secure.symantec.com/connect/videos/group-update-providers-part-1
https://www-secure.symantec.com/connect/videos/group-update-providers-part-2
I hope it will help you !!!