I hope Sweeneys waits for a deal to be made. Ullmark is the best goalie who is available. Detroit, Colorado, Ottawa, Carolina, Toronto still needs to upgrade their goaltending. There could be a bidding war for Ullmark very possibly. The only problem is Ullmarks NTC ...
If you're Ullmark, for the best job/paycneck security you should want to waive now to a team offering an extension. You get to control where you land(out of the current suitors), you get the comfort of knowing you'll be there for X# of years.
If you ride out the year in Boston you run the risk of injury, of a bad year stats wise killing your value, or even the team you may want to go to in FA ends up solving their goalie issue leaving you a bad pick of possible teams.
The one reward would be having another lights out vezina caliber year with Boston means increasing your $$$ a bit come next offseason.
Would be too risky for me.