Tetrahexyldec stimulates more collagen than L- ascorbic acid so you get more benefits with anti-aging, plumpness and firmess. Also there is more collagen production with this particular ingredient. Tetrahexydecyl is a potent anti-oxidant which tends to give superior protection against free radical damage and prevents lipid peroxidation where free radicals damage your skin causing photoaging and black heads.
Another factor that it does better against L-ascorbic is that it is more stable therefore it won't destabilize itself quickly. Lot of L-ascorbic in the market tend to oxidize before it reaches it's shelf life.