It's my understanding that the TSW versions were made to be somewhat higher quality and a bit more refined than their predecessors. Similar to the many of the differences between the standard and Police/Wingmaster Remington shotguns.

Of course, that may just be S&W's marketing tactics talking.

There are some noticable differences besides the accessory rail, though. The frame rails are different (more robust, with tighter tolerances), and they feature a delayed-unlocking firing system, both of which supposedly gives them a slight edge in accuracy and reliability.