Whitter only has 1 train/ container barge 2 times a week the last time I worked on that dock. It is great for deep water but no dock facility to handle the quantity of containers. If it was big enough the trucks would be bumber to bumper 3 days a week going to anchorage . After they get through the tunnel.  Lots of restrictions there.   That is not open 24 hours a day. Now to talk about the weather , the wind can be in excessive of 80 mph, and can tip over the container crane. (Happened in Dutch Harbor).
Seward very good option dock wise. Traffic would be all the way to Anchorage. Trains could pick up more freight but you would need a much larger train yard in Anchorage. Truck traffic would then increase wherever that yard would be.
Valdez good option room to grow. The bad: No train, Thompson pass,  traffic down the Glenn after Eureka. 
You also have to take into account that the turnaround time on offload and load is less than 20 hours? 
I will continue more.