BULA Combo Pass/ Diving Isands

My friend has just purchased the combo Bula pass and I have decided to join her, but am torn between buying the island pass or the combo pass. My concern would be I would not be able to book the same accomodation as her when the time came. Any advice please?
Also, am keen on getting some dives in, hoping to see some mantas. My friend is not a diver, so would need a compromise of places to visit over our 10 days so we can do a mixture of beach and water activites. If anyone can suggest which islands stand out in this area me I'd appreciate it.

Thanks.

Amanda.