The answer to this will depend on your school or organisation and how you use your minibuses.
Two 9-seater vehicles will carry 16 passengers, but will need two drivers. However, anyone with a car licence can drive 8-passenger seat vehicles, so you may have more staff who can drive them. If you have two vehicles, you can take smaller groups to different places, rather than sharing a single vehicle.
Two vehicles may cost more in terms of maintenance, fuel, and insurance, so you will need to do a cost analysis on both options. Rivervale Minibus can help you with this to decide which is the better option.