If it were me ...
If I had the choice of choosing between an older 5906 and a 5906TSW, I'd opt for the TSW.
The TSW included some nice enhancements, including beefier box-type frame/slide rails, delayed unlocking and a loaded chamber inspection port.
The thicker frame rails were designed to help aid potential accuracy, and the delayed unlocking was claimed to reduce felt recoil.
We were told that in one point during 5906TSW production, the chamber wall angled received a 1 degree change to enhance extraction (if "variable" tolerances of case dimensions were encountered and made extraction "sticky"). This change was supposedly identified in the barrel revision where the caliber designation on the barrel hood read 9MM in large alpha /numerics, versus the previous 9mm Parabellum in small alpha/numberics. The barrel hoods were thicker in the TSW's, too.
The revisions and improvements in overall manufacturing were occurring pretty often, and the TSW's benefitted from all of them. Better drawbars and more cleanly machined inside surfaces, as well as the enhanced MIM triggers, sears and hammers.
The plastic/nylon disconnectors and mainspring plungers (cups) were good things, too.
I remember when an armorer instructor told us (in one of my 3rd gen pistol armorer classes) that before changing from stainless steel to plastic disconnectors, the factory conducted endurance testing on the plastic disconnector concept. The plastic ones demonstrated durability at least rivaling that of the steel ones, and the testing went out beyond 25,000 rounds, we were told.
The consistent dimensions and tolerances on the plastic ones were better than the older steel ones, too. The problem described in an older armorer manual of "long tail disconnector" was no longer included in the subsequent manuals, for one thing. Also, the inherent reduced friction of the plastic part resulted in less draw against the drawbar's disconnector tab.
The edges of the plastic mainspring cup made for a smoother stacking of the mainspring, too, as the individual coils rubbed much more smoothly against the lip than in the older metal cups. A minor detail, but many years ago you could rear of how some smiths offered radiussing/smoothing of the metal cup edges to help DA trigger feel, so the coils didn't catch on the sharp metal cup edge during compression. (My finger's not that sensitive, I think.)
I actually replaced the steel disconnectors with the newer plastic ones in my own issued 5903/6906's, and most (if not all?) of my own personally-owned 3rd gen guns. That was many years, and many thousands of rounds ago.
Also, the newer MIM sear was called (in-house, we were told), the "super sear", because it was made to very good tolerances and could be used in both new model TDA (DA/SA) and DAO models that used the new MIM parts. One of the factory guys told us how the original, and very expensive, MIM molds were made using language to ID the MIM sear's application before the engineers had finished designing the new style DAO, and realized they could now use one sear in both designs. (older DAO guns used a designated DAO machined sear.) Hence, the in-house reference to it being a "super sear" , even if the language of the first mold no longer applied.
I never cared for the riveted accessory rail, myself, but the later production 5906TSW's could be ordered with the machined integral frame rails (full-size models in all 3 calibers could be ordered that way, at no additional cost, during the end of LE production, BTW).
There's probably various other minor things I've forgotten since those days (I took 4 classes for the 3rd gen guns, and had a lot of phone conversations with various guys at the factory while supporting them as an armorer), but those are the things that stick out in my memory, off the top of my head.
Oh yeah, as the guns were being made with new CNC machines, and some laser cuts, the inside of the frames and slides were benefiting from much cleaner cuts and surfaces. Even the barrels of the TSW's were benefiting from some ongoing revisions right up till the end of production. I saw that exhibited in some 4566TSWs during the last couple year's of production.
) there are some subtle ergonomic improvements also, a big one for me is that they did more detail rounding and de-horning where the bottom of the trigger guard meets the front of the grip.
