Seattle-Tacoma International Airport has two Delta Sky Club locations - the main Club located next to Gate A1 that primarily serves Concourses A and B, and a smaller Club on the second floor of the South Satellite.
I think the A Club is one of the best in the entire Delta system. It takes approximately 10 minutes to get from the A Club to the South Satellite via the Satellite Transit System (an underground train). If you have a long-ish layover, or even if you are initially departing from the South Satellite, I suggest taking the extra time and utilize the A Club. If you have a short layover, the South Satellite Club is suitable, but it is not on the same level as the A Club.