They should just blow him away with a top of the market offer and be done with it. If he doesn't accept, move on.
If they keep Betts all year, they get like a 4th rounder as compensation I think. If they trade him midseason, you'll get a far smaller return.
To me, trading him at midseason is a bigg gamble. What if he gets hurt in May? What if you are still in contention in August? If the Sox stink from day 1, are they going to trade him early and drive fans away in June? Far too many variables for my liking.