The main differences are in lifetime and safety. If there are children or anyone who is a bit frail or clumsy about, polycarbonate or safety glass is recommended. Polycarbonate will eventually become less clear or crazed after over the years, with useful life depending on the quality of the UV stabilization used. Expect 10 to 20 years. Standard greenhouse (annealed) glass will stay clear effectively forever, but produces dangerously sharp shards when broken and does become more brittle over time. Hail can be a problem. Safety (tempered) glass is much safer, but also more expensive. If you will be heating the greenhouse, look at multi-wall polycarbonate panels, as they have greatly increased insulation properties over glass (although extremely expensive double-glazed glass panels are available, too.)
