Living in ANC my experience is that the SEA flights on the 737-800/900 tend to work very well for inflight wifi. (Actually GoGo works well on both DL and AS for this route in my experience). I'd say 95% of my flights on this route have had no issues. The wifi has gotten better when I take the MSP flight, but to me it's not as reliable on that route. I'm doing the summer nonstop ATL later on this month and I'm expecting it to be about the same as MSP or worse since it's the old domestic 767.