Travel length
Up to roughly 6 metres unsupported, plastic is nearly always the best choice: lighter, quieter and cheaper. Beyond that sag starts to matter and steel with support rollers becomes attractive.
Fill weight
Above 10 kg per metre of fill, plastic reaches its limit on long travels. Steel easily carries double and does not deform permanently.
Environment
Hot swarf, weld spatter, glowing particles or aggressive coolants: choose steel, optionally with a swarf cover. Plastic handles roughly 100 °C continuously.
Noise and speed
At speeds above 3 m/s or in rooms where people work, plastic is markedly quieter. If steel is still needed, choose a version with plastic glide elements.