A hard choice... That unfortunatly we probably won't have to make...
Grant seems to be exactly the right guy on the target definite by Ainge. Both have great good contracts for the next 3 years, but Grant still 5M less, which is not negligable.
Vucevic would give something worked very good under Brad with a technicly gifted center, ala Horford with less defense but more shoot. But at center we are already loaded and Rob would't have room anymore. At 4 we have more needs. Finally I prefer Theis at the 5, even with his lack of size or verticality it is there that his versatility is the more usefull. Grant would perfectly space the floor and give us a terrible 3 wings rotation. As he doesn't need as much the ball than Hayward, it is less a problem in the plan of giving more and more the game on jays hands. Grant showed how effective he can be as a third offensive option in Houston. And Grant is also in a team who don't play anything for the next 3/4 years unlike Orlando who could wait to see how an Isaac/Vuc combo would give. Grant is also younger and Vuc may begin to regress during his contract.
So finally I think I would go for Grant; if the price was the same. But both would be great additions, and would on my advice put us again in the same list than Bucks, Nets and 76ers to go threw the east.