I'd keep Stills, because he's been more consistent than Parker. Parker is the guy I'd look to trade. But be careful for what you wish for. Both Grant and Wilson may not be 100% healthy yet. The Dolphins are in a unusual position. If you trade Parker you can keep both Butler and Ford on the 53. Both offer different skillsets. With the 2020 draft being so deep and very talented at the WR position that be when I'd entertain the idea of trading Stills.
Stills is worth considerably more in trade than Parker, at this point. It's apples and oranges.