Dave, my experience from several friends conservatories is this. Also my own observations from an engineering point of view and personal taste.
Facing south your will get a lot of direct sun unless you are close to trees etc.
Glass is the best option. It will be double glazed and have that magic K factor that the government has insisted on for reduced losses of heat ,this K glass appears to give that slightly uncleaned blue /grey haze effect on the glass.. It does not reduce the heat that builds up during the summer months though so you will need to fit one of those ceiling fans.
So you adopt the double glazed glass and then you spend another £2000 if your not careful in costs for the blinds for the roof. Your wife will also insist that you have vertical swivel blinds for the side windows.
The polycaronate roof is more prone to crack with snow cascading off of the house roof a common cause. Not sure where you live Dave but down in sunny Berkshire we don't get much snow but it may be worth considering.Also Polycarb does look tatty much quicker than glass
and ,glass does not scratch and is much more weather
resistant. The verdigris picks up very easily on polycarb as well and I don't think any window cleaner will be interested in cleaning it either.
PS regaring the roof vent you can get an automatic one and if it rains it auto closes as well.
So Dave my verdict is GLASS.