If it were me, and I planned to do a LOT of shooting, and I had the choice of either a standard 4006 or the 4006TSW ... I'd opt for the TSW version, even if it cost more money.
The TSW's benefited in a few ways. Larger box-type frame/slide rails (helping to contribute to durability, tighter tolerances and better inherent accuracy); Loaded chamber inspection port (handy); and Delayed unlocking (might feel a slight reduction in felt recoil).
Overall better machining and tighter tolerances, including some revisions, such as a longer ejector (for faster ejection & curved angle underneath tip for reduce potential of a stress riser - broken tip); revised extractor (with an added machined bevel under the hook for enhanced feeding of case rims -which can vary among ammo brands and even different lines within the same brand).
The optional integral machined accessory rail was an improvement that I felt was a bit late in arriving in the full-size guns. It was offered as an option to the riveted rail at no additional charge for LE agency customers when ordering the guns. I imagine some LE trade-ins will continue to hit the wholesale market as 3rd gen guns are replaced by other newer, less expensive options in coming years (unless local laws/ordinances require destruction of out-of-service duty weapons, of course).
FWIW,
one of the reasons some older 4006's were replaced with TSW's in LE service was that S&W no longer made the older 3rd gen .40 barrels, and the .40 TSW barrels are different enough that they aren't interchangeable with the older 4006 barrels.
The current production (for existing LE agency orders) TSW's are pretty much the best of the bunch of the 3rd gen's ever produced.
The current 4006TSW is typically considered to be stronger and more durable than the original 4006 model.
