I would always say glass as its fairly cheap to get, aslong as its horticultral and toughened, although im not sure if this is 100% necesssary. Gibbs and Dandy (there are other suppliers available) do glass and will cut it size, having known this as our dog ran through the first pain put in the frame a decade back! But she was ok! I think glass lets more light in, poly tends to be slightly 'cloudy'.

As polycarbonate has a top and bottom layer and is 'hollow ' for want of a better word, I would imagine that algae and dirt would accumulate in it.

I prefer glass to polycarbonate, also (note to self - need a sheet asap!). The glass is much clearer than polycarbonate and lets in more light.
We have a polycarb. frame that is useful for plants that need summer protection but shading from the hot sun. Also several sheets of polycarb. that I use as temporary or emergency covers. We have had these for a number of years, Nicky, with no eveidence of dirt or algae in the hollow tubes. It does become more opaque over time.

Given the chance to choose again?...I'd have glass panes instead of polycarbonate.

When they make a polycarbonate you can clean and polish and that doesn't deteriorate in the weather, then I'd say polycarbonate because it is a better insulator.
Until then, stick with glass!

Totally agree! In my case, hindsight is a wondrous thing! LOL

poly scratches easy and as nicky sais starts going opaque quite quickly and thow it seams to let light through still it cuts out some of the more important spectrums that plants need . even water does this however clear it is .