Moclobe how are you the only place i know of that has track for 6mm sliding doors is lincoln sentry in geelong www.lincolnsentry.com.au they have other stores also but i am not too sure of locations.
Ether the difference is that 6mm is usually laminated so doesnt shatter when broken, where as 5mm is either standard glass (shatters) or hardened (expensive)
In my experence I have had no "cons" in using perspex, at least for pythons. If it has better heat retention than glass it is marginal.
The big advantage is in the display cages that I use for shows. I know that the perpex is not going to crack or shatter during transport.