It shouldn't be necessary to disable
systemd-timesyncd if you install
ntp, because the
ntp service configuration includes the following:
# grep -i conflict /lib/systemd/system/ntp.service
The same applies for the
chrony service on newer Ubuntu versions.
There have been startup bugs in the past which meant
systemd-timesyncd tried to adjust the clock while another NTP server is running, producing harmless-but-annoying errors in the system logs. So there should be no harm in disabling
systemd-timesyncd if that's what you prefer.