My suggestion is to meet the Instructor face to face and choose the one you feel most comfortable with.
There are many good Instructors on the island and the best scenario is to take the time to walk into the shops and meet the Instructor.
Harriete resort dives would be an extremely expensive option as resort dives are $125-$175 each and there is no certification involved.
The Refferal is anywhere from $200-$275 depending on the shop and includes 4 dives and the certification paperwork so its the best option in this case.
All the best
Gaz Cooper's DIVE BELIZE www.divebelize.com