For a panel of the same thickness, bog-standard cheap polycarbonate lets in less light. It also lets out more heat, so it has less of a greenhouse effect. But you can buy special polycarbonates which are specially designed to let in more of the light of the most important wavelengths, though maybe then your cost-saving is gone. On the other hand, the most important effect of a greenhouse is a wind-shelter, so these differences are perhaps not so important.

But polycarbonate for greenhouses is often double-skin, which acts like double-glazing, but at a fraction of the price. This lets out much less heat than single-skin glass, so keeps things warmer. But also lets in even less of the light in the first place. So the balance of advantage depends upon how important high light intensity as opposed to maintaining warmth is for the plants you want to grow. The usual solanaceae (tomatoes, capsicums, aubergines) and curcubits would probably prefer the glass.

Finally, whatever they say, polycarbonate always gets less transparent as time goes on - UV browning, whitening from flexing.

For my own garden I intend to buy a tempered glass greenhouse, when my finances recover from the house extension. But I'd get a polytunnel for an allotment.