The main difference is perspective, which is controlled by camera-to-subject distance, not cropping. If you use a wider lens and move closer to keep the subject the same size in the frame, facial features can look exaggerated or distorted. That’s why portrait photographers often prefer longer focal lengths and a bit more distance, which usually gives a more flattering look.

Cropping a 35mm shot to match the framing of a 50mm or 85mm shot does not change that perspective. It only throws away part of the image, so you lose resolution. Cropping also won’t recreate the depth-of-field look you’d get from using a longer lens from a different shooting position.

So:

• Moving closer with a wide lens changes perspective and can add unflattering distortion.
• Cropping keeps the same perspective but reduces image quality/resolution.
• Using a longer focal length lets you frame the subject from farther away, which is often preferred for portraits.

Also, 50mm is not a universal portrait choice. Many photographers prefer longer focal lengths still, often in the 85mm-and-up range depending on sensor size and portrait style.