The length is not the only factor to choose a board, stiffness plays a big role also. Also, depends on the type of riding you intend to do. My eyes were opened when I changed from 164W stiff board to 157 loose (I'm 6'4/180lbs), I mainly ride small hills with small kickers. If I went to ride bigger hills/mountains, I'd probably take the bigger/stiffer board.
Length does matter, but buying a longer board without a real purpose (e.g. deep powder, megafast bombing...) is not a reason to get a new one. Even the graphics are a better reason IMHO