In systemd logs are stored in an internal binary format (not as text files) so you need to use journalctl command to access them:

sudo journalctl -u service_name
sudo journalctl -u service_name -f

In upstart logs are normal text files in the /var/log/upstart directory, so you can process them as usual:

cat /var/log/upstart/foo.log
tail -f /var/log/upstart/foo.log