Sram cranksets, are there differences other than weights betwen models?
The major difference is the materials, such as steel or aluminum chainring, the level of aluminum used in the crankarms and the way they are machined. All of which does lend to the overall weight of the crankset. See below for a side by side comparison as well and pictures of each in which you can see the difference in the chainrings and the amount of machining in the backside of the crankarms.
Notice the NX and GX use the aluminum XSync 2 chainring, while the SX uses the Steel Eagle chainring. All of which have the same tooth profile to run Eagle chains.