Tsao's probably still considered a top prospect, but the fantasy outlook for him has always been relatively bleak, since he'll be pitching at Coors half the time. Anyway, his talent will not translate to optimal stats, but he should fare pretty well as the closer. I expect him to win the job outright from Fuentes (LHP) pretty soon. He can really bring the heat, so he's definitely worth a later round pick at the back end of your bullpen.
He was moved to the bullpen due to the injuries he's suffered and to minimize future injuries.
Chacin will be fighting for the #5 SP job in Toronto. His stats have always been impressive in the minors, but due to his lack of real heat with the fastball, he doesn't project as anything more than a back of the rotation starter. Even as a closer in Colorado, he projects as a better pitcher. His stuff is light years ahead of Chacin's...if they were actually in similar situations as starters in fairly like parks, this would be a KO.