The 6.8 Ah battery has 244 Wh, assuming you're running it at 20 km/h for 8km, consuming 350W all the time (which is highly unlikely), you'd be needing around 140 Wh, so if you're only using the motor on one leg of your commute, you should be fine with the smaller battery.
The 6.8 Ah battery has 244 Wh, assuming you're running it at 20 km/h for 8km, consuming 350W all the time (which is highly unlikely), you'd be needing around 140 Wh, so if you're only using the motor on one leg of your commute, you should be fine with the smaller battery.