In general, I think that the diving close-in to Taveuni is better and more diverse than the close-in diving to Savusavu. Namena is the exception, of course, but it is a long boat ride from Savusavu and is not a regularly scheduled trip for any of the 3 Savusavu operators that do go there. There is one resort on Namena Island (Moodys Namena) and they dive exclusively in the marine reserve, if you want to have maximum exposure to the area.
Thats not to say that Savusavu doesnt have some great dives, but Taveuni offers a fuller spectrum of dive types that are less of a boat ride away. Of course, which resort you stay at will also have a bearing on the length of your boat ride to a given site. I think one indicator of the relative desirability of the two areas is that while the closer Savusavu operators make an effort to go to Somosomo, no one on Taveuni goes to Savusavu. I think that the Taveuni operators might, however, consider going to Namena if it were not so far away. I believe that the best drift dives (and probably the best access to pelagics as well) are Taveuni and Namena, but there is good macro at all 3.
So to summarize. In my opinion the biggest reason (but not the only one) to stay in Savusavu would be to dive Namena, while getting similar quality (to Namena) at Taveunis sites is more likely. Weather, currents, and other factors can be an issue at either area that might limit your access to the best sites. I would rate Somosomo and Namena roughly the same, and Savusavu slightly lower in desirability.