For perspective...Last year I re-did my deck at the NJ Shore with composite decking, it looked (and still looks)terrific, but I put it on 16" centers and it did sag(even without anybody walking on it). Secondly, even though it's plastic...it is still quite pourous and it gets dirty and...it takes a power washer to really clean it. It is also quite heavy(much heavier than comparable sized lumber) and difficult to drill.
I would think that if you wanted to do something beautiful, different and long lasting consider Mahogany (actually less expensive than some composites).
Good luck
