Although the BAC-601 Display unit provides a digital speedometer, it does not allow you to modify any of the controller parameters during a trip like the Bluetooth application does.
The biggest problem I find with the BAC-601 Display unit is that it does not retain any of the trip data when the battery is turned off, the odometer is the only information that isn't lost.
If you are using a pedelec sensor, the separate mode selection button unit that comes with the BAC-601 is probably easier and more convenient to use by feel alone instead of having to look to see exactly where to press on a phone display.
Because the BAC-601 Display unit and mode selection switch is permanently fitted to the bike, you will never set off on a trip without it. Whereas with the Bluetooth option, if you go for a ride without your phone, you will not be able to adjust the level of pedelec assistance or log any details of the trip.
Alan