I do not recommend any synthetic decking at this time as there are really not many out there that have passed the tests. Trex is a very good example of this problem.
Now, as a deck contractor, I recommend joists spans of 24 inches if using 2x6 or 2x8 decking. Now, most composites are much thinner than this 2x material. Even the 5/4 PT wood. Therefore I would recommend 16 OC with stuff smaller than the 2 by material.
I personally hate a bouncy deck or one that bends when you walk on it. On decks that I build with 5/4's for example, I will go 12 OC regardless what the manufacturer recommends becasue I have to answer to the customer or the one making out the check.
Synthetics are advertising heavily and if you know why, they are rushing in to be available and available in your memory because Pressure treated wood becomes discontinued next year. so everyone is rushing to have a market share of the void that pt is presenting. Not allot of testing is being done and in my opinion, a synthetic deck needs to be tested for over 10 years before it can make all these claims that they are doing.
Its your call and your money. But, take heed to Trex's problems and this could very easily be all the syn problems too.