The difference between Full Flex and First is usually relatively reasonable, and certainly I don't think the relatively small difference is worth using 15,000 Alaska miles each way for--you can get a much better value than 1 cent per mile. (You're also taking a chance that it U space will never open up.)

You might consider just booking First. For the extra ~$150 each way, you get the a 50% bonus on redeemable miles, Board Room access, and two free checked bags per passenger (normally $20 each).

If you don't plan on spending the money for First, I wouldn't book Full Flex. To use a mileage upgrade, you actually only need to book Value to be able to apply a mileage upgrade. Value fares are hidden by default; to show them, make sure to choose the option for "Mileage Upgrades" in the upgradeable fares menu to the right of the search screen.

Now, a question of my own: is it possible to book a Best Deal fare and then--if U space opens up--later "upfare" to Value or Full Flex? I know it is possible to do a full upfare to full-fare First, but is it possible to upfare to anything less than full F?

(If the answer is yes, I would suggest booking Best Deal and then calling in to upfare to Value if U space opens up when you want to use your miles. This way, you'll only pay Best Deal if the upgradeable space never opens up.)