I think they will defintely be a mix of 739ER and 738. From what I know the 739's are always filled up for the florida runs. It would make sense for them to order 739ER for florida, LAS, and hawaii runs. Although the 738 does it great, these a/c could be used in conjunction with air mike.
Now why didnt order the 789 is not my concern. I just hope they're happy with the 788. Any chances of ordering the 783? If they did it would useful for runs between hubs and florida, and maybe hawaii where there is cargo demand. Just my opinion.