When it comes to polycarbonate versus glass greenhouses there seems to be a lot of confusion for many people wishing to purchase one of these great gardening buildings.
(Me included at first!) Here is a guide I have complied on based on my own experience combined with that I have found across the internet.
There are many pros for both Polycarbonate and Glass.
See below.
Pros of Polycarbonate

• Twin walls for better insulation- helps to keep temperature constant
• Ribbed channels- These break the suns rays helping to reduce the need for shading
• UV protected by diffusing light
• Shatter resistant
• Lightweight and easy to move if needs be
• Supplied in exact lengths needed when ordered in a greenhouse kit
• Heat loss decreased due to twin walls
• Flame resistant
• No condensation
• Cheaper to buy and install
• Better protected against vandals and children's ball games
• Can cut panels easily at home with a Stanley knife

Pros of Glass

• More aesthetically pleasing
• Better light transmission
• Stronger against the wind
• If it doesn't get shattered its longevity will outlive that of a polycarb
• Flame resistant

According to pros, polycarbonate seems to have more features that attribute to a good, long-lasting greenhouse.
A glass greenhouse probably looks nicer and will last longer but only if it does not get hit and shattered first! If this does occur it'll be costly to replace a glass panel compared to a polycarbonate panel.
Here are the cons of a polycarbonate greenhouse compared to the cons of a glass greenhouse.
Let's see how they compare: Cons of Polycarbonate

• Can warp over time and panels can fall out
• Can get scratched

Cons of Glass

• Up to 3 times more expensive than Polycarbonate
• Can't cut out panels as easily at home
• A glass greenhouse will require sealing which is expensive
• More difficult to transport during delivery

It would seem that on pros, polycarbonate wins again.
There are more cons for glass and more pros for polycarbonate.
It all of course relates to your own individual requirements but I know I'll definitely be going for a polycarbonate greenhouse in the summer.