The Starwood AmEx is a better card than the Alaska Visa for general spending. While the Alaska Visa is good for the companion ticket, it's not a great card for general spending.
Because 20k SPG points = 25k Alaska miles, I essentially earn 1.25 Alaska (or many other airlines) miles per dollar spent on my Starwood AmEx whereas I would earn only 1 Alaska mile per dollar on the Alaska Visa (and I can't move those miles to other programs).