I own a deck building company and we have worked with TimberTech and Azek, amongst other products, over the years we've come to like some products more than others.
Azek is a PVC decking material. Timbertech also manufactures a PVC deck board called XLM - TimberTech also manufactures five or six other types of decking products.
Most consumers are familiar with traditional composite decking - first introduced by Trex in the mid 90's. Traditional composites, in very general terms, are a combination of wood and plastic. Although they are "low maintenance", they have their short comings. Since composites have wood in them, they will absorb stains like grease, coffee, wine, etc. Some composites will fade, some scratch easier than others, some get extremely hot, and others expand and contract at considerable rates.
PVC products offer stain resistance, fade resistance, scratch resistance, and they come in more color options than composites - which is why we typically recommend PVC decking.
We prefer the Timbertech PVC product, called XLM. It is stronger than Azek (we have to frame Azek at 12" o.c. to account for its sponginess), it has a far superior hidden fastening system, it has a class "A" flame spread, and we get better customer support from Timbertech than from any other decking manufacturer - and we deal with them all. XLM has performed very well for us - we've a lot of happy customers that echo our sentiment.