A lot depends on what it takes to get a particular guy, but even then, your comparison isn't apples to apples with Skubal being a FA after the season and Miller having multiple years of control remaining.
Overall, I think you benefit more from getting a good SP who can start a playoff game but the calculus depends a bit on who's coming back healthy and what role they're going to have. This team doesn't have enough pitching, period. Add Cabrera and a trade piece to the rotation, then get Palencia, Brown, Harvey and maybe Steele back in the bullpen and the need becomes less, but you're not going to know that until after the deadline so there's some guess work involved.