So many options. Fiji is great for learning to dive. Don't worry too much about that part, but look at the destination in a more general sense. Where as diving for experienced divers is more specific, in your case, I think you have many choices. I would not normally recommend the Yasawa islands for divers, they do offer some classically wonderful islands with very good diving especially for beginners. I can;t argue against Taveuni at all but it is further and more expensive as a consequence which may or may not be a factor. Have a look at Beqa Lagoon Resort for a more accessible option with the bonus of a shark feeding dive on site. If budget is an issue, try Aquatrek for diving and stay at either Uprising or Club Oceanus. Volivoli or Wananavu might offer learn to dive packages and host some of Fiji's best dives as well at an easily accessible location.
Bottom line, you can't go too far wrong. Pick a place that suits your budget and has good reviews.
PM me if you want any more specific info.
Greg