Where are you leaving from/returning to? Is it the U.S.?
When you plug your dates & destination into Orbitz, Travelocity or similar, how do the plane trips compare in terms of # of flights, flight times, layovers & cost? How long does it take you to get there? How long can you bear to plant your butt in a plane seat continuously?
Is the trip all about the diving? If not, what else do you want to do?
How did you arrive at Fiji as the sole alternative to Roatan?
When you check into Roatan, you'll find in other threads that the island has different regions, and your stay at different parts & resorts can give you a much different experience. Many people love CocoView, so I recommend checking that out for an 'all you need on the resort grounds, whole lotta diving in a week' trip. Get your cost, travel arrangements, etc...laid out. Read some reviews.
With that as your yardstick, then pick a popularly recommended resort in Fiji, and start doing the same thing.
Richard.