Run44:
Are you planning to actually fly ON Alaska Airlines somewhere in the states? If you are MVP with them, you have a good chance of getting the upgrade. If you are not flying to Seattle of Alaska, I would recommend sticking with Delta.

Also know, once you start any portion of travel with Delta, they will not let you change your FF# on any remaining flights. In other words, even if you have 4 more flights attached to the reservation, months down the road, they will not change the FF# that was input originally.

I would probably recommend Delta, just based on their sheer size.
However, the customer service from Alaska is superior and they treat all of their customers like kings. I love Alaska. I wish I still lived in the PNW.