Not much insulation I'm afraid, from either. Polycarbonate is slightly better because of the twin-wall structure with air in between. Single glazing is very poor. Using "low E" glass is better, but no comparison with double glazing.
Most conservatories are not constructed to carry the load of glass, certainly not double-glazed units. It depends on how the structure is built.
If you have to use Poly, then maybe go for this...............
http://www.insu.co.uk/conservatory-upgrades/polycarbonate-roofs/
It's a new one on me, but it certainly looks like an improvement Ratts.