Hi,
We would recommend GUP only because clients are only 10-20 per sites.
As pete said it will ease the administration. You can manage all remote sites from the SEPM sitting at main site.
It's not recommended to install new SEPM for 10-20 clients until and unless it's really required.
Group Update Provider: Sizing and Scaling Guidelines
http://www.symantec.com/business/support/index?page=content&id=TECH95353
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