A bit confused here. Just started looking into third gens. The 6906 is just a sort of compact 5906, correct? Is the frame on the 6906 alloy or steel? Any variations in sights? I have seen pics of them with the eared, Novak style adjustable rears and other pics without those.
I'm sad to say that I don't recall where I stole this from, but you may be able to google to find the original source (opinions are not necessarily mine, just part of the text I had saved off):
There are 4 basic variations of the Model 6906, in order of decreasing age and roughly increasing desirability:
1. Square trigger guard, vertical blade rear sight
2. Square trigger guard, slanted Novak Lo-Mount rear sight
3. Round trigger guard, Novak rear sight, silver-color flash chrome trigger, silver-color flash chrome solid-sided hammer (last two also used on all earlier guns)
4. Same as #3 but with black MIM trigger and black MIM "skeletonized" or "cutaway" hammer
The early vertical blade rear sight is OK but the Novak is better and has more aftermarket replacement options (slide dovetails are different sizes so they don't interchange).
The square trigger guard does not fit in all holsters and is ugly to boot.
The later MIM hammer and trigger yield a better out-of-the-box trigger pull, particularly DA, and some S&W pistol fans prefer them; yes, you read that right, this is an instance where shooters think MIM is actually better.