The resolution for the laptop screen should be set to 1366x768.
The external monitor will depend on how it is being used. If you are cloning the desktop (ie: same image on laptop screen and external monitor), then they will both be the same resolution (1366x768). Thus the ext monitor image won't be ideal. If you are extending the desktop (different images on both screens - one big desktop) to the ext monitor, then you should change the resolution on the ext monitor to 1920x1080 (assuming the laptop is powerful enough to push two screens at that resolution).
The external monitor will depend on how it is being used. If you are cloning the desktop (ie: same image on laptop screen and external monitor), then they will both be the same resolution (1366x768). Thus the ext monitor image won't be ideal. If you are extending the desktop (different images on both screens - one big desktop) to the ext monitor, then you should change the resolution on the ext monitor to 1920x1080 (assuming the laptop is powerful enough to push two screens at that resolution).