For me, I would only trade Sanchez or Stro' if we received a top AAA starter who could possibly be very good, #2-3 in a rotation, plus some low level kids.
But if we hold on to both, to start the season, and it's late June and both are pitching like they can, I think we get a lot more. I think even if only one of the two pitches really well (say Stroman), the prospect haul for just a good Stroman would be more than trading both of those guys away now.
Players like Pillar and Morales are the opposite. They have almost no chance to enhance their value after a couple month of play. They are in decline.