I saw inside a taxi recently that the driver had about four different gadgets arrayed on the dashboard to do his job. There was navigation, the dispatch system, credit card reader and the fare meter. It seems like those could all be integrated into one big taxi management device, and there might be some advantages to doing that, but there would probably also be some disadvantages too. For instance, if the credit card reader hardware is found to be insecure and needs to be upgraded, an integrated system wouldn't allow that. You'd have to replace the whole thing.
So if your components are liable to be upgraded individually, modular design is best. But sometimes the benefits of integrated systems will outweigh that advantage, especially if it's small. The overall cost of an integrated system should be lower, it takes up less space and can be designed with one smooth interface.
Mokalus of Borg
PS - I think the navigation, dispatch and fare meter systems could be done in one app on a dashboard-mounted iPad.
PPS - Then I have another idea for payments I might talk about later.