Hi Holger,
The main difference is between the Rds(on) of the two devices.
The DRV8802 has a higher Rds(on), as shown below:
DRV8802 Rds(on) Spec:
The DRV8814 has a lower Rds(on), as shown below:
DRV8814 Rds(on) spec:
As shown above, the DRV8814 has a higher Max current specification in comparison to the DRV8802. That is the main difference between the devices.