I fly SEA OAK, OAK SEA a lot, and have found that chances of upgrades depend on the time of day, the evening flights for weekdays definitely fill up either by paid or upgraded MP members while morning flights OAK to SEA seem to be wide open based on the number of available seats. So, it is not just the route, but also the time of day when there are several flights to an airport in one day that makes a difference.