I would like my fellow FF's opinion about using certs or miles for upgrading. It seems that when booking a flight using miles gets UA's attention to confirm the upgrade more often than using the cert. Is this impression generally true? thanks

When flying domestically I would prefer to use the cert since I have so many and the milage impact on my account for an upgrade is considerably more higher (e.g., 10000 miles vs 5 500mile certs).