Hi
Are you going to hire a car? If you are anywhere on the southcoast would be a good base for getting around to see the sights of Upolu. As for diving, you'll need to contact someone like Manoa Tours who are located on the south coast and specialise in water sports. They pick up from most locations to go out to the reef.
On Savaii it's a different story. If you stay at any of the fale properties at Manase Beach (Tanu's, Janes, Vacations) you'll be able to go out to the reef yourself for a dive although getting around without a car could prove difficult unless you ask one of the accommodation providers to provide you with the name of a reliable taxi driver to take you around. Sam is one that comes to mind, he does tours too, unfortunately I don't have his contact details.
To check out your accommodation options further go to samoahotels,ws
Deb