Usually, if one setup fills more of the sensor with the slide, it wins unless the other lens is dramatically sharper. Cropping reduces effective resolution because you are enlarging a smaller captured area.

A useful way to think about it is lens resolving power: after cropping/enlarging, the effective detail goes down roughly in proportion to the crop. So a sharper lens only helps if it resolves enough extra detail to overcome that enlargement penalty.

In your case, the question is whether the 50mm prime resolves roughly 30% more detail than the zoom at the needed framing. That’s possible, but often not the case, especially if both lenses are stopped down to a good aperture. Manufacturers rarely publish directly comparable resolution figures, and third-party tests may use different methods.

So the practical answer is:

• if the zoom lets the slide fill the frame, it will often be the better choice;
• stop both lenses down to their sharp range and compare actual results.

Best approach: make a side-by-side test of the same slide, matched for exposure and focus, then compare fine detail at the final output size. That will tell you more reliably than specs alone.