I think that the TSA website is causing undue confusion with their acronyms. From what I know, there are three security checkpoints - north, central, and south. North serves the D and N gates, Central is for the B and C gates, and South is for the A and S gates. I have found that the North checkpoint tends to be the most crowded. I personally like the South checkpoint.

If you would rather spend your time walking than standing in line, I would pick either the Central or South checkpoint, no matter which gate you are going to. Any checkpoint will get you to anywhere within the secured area. The N and S gates require you to take the underground subway, which is on the secured side of the airport. The rest of the gates are a 10 minute walk from almost anywhere, with the exception of the low numbered A gates which are quite far down towards the south.

During the past year, the most time I have spent in a security line at Sea-Tac is about 20 minutes. I would say that 10 to 15 minutes is typical. I wouldn't worry too much about it.