There are frame rails that raise the floor next to the driver and passenger seats. The kick panels in a coupe come down about 2-3 inches further, so there will need to be 2-3 inches more carpeting in the front on the sides. Since the carpeting often has to be trimmed, this may not make a difference, but there may be a difference in the way it is moulded. I do not know of any other differences off the top of my head.
as mentioned, convertible have inner rocker panels unlike coupes or fastbacks. so, the convertible carpet is molded to fit around it. you cant use a coupe carpet in a convertible, it won't fit. another difference is the transmission tunnel, convertible have a reinforced section with a double hump.