I'd actually go the other direction in the 6906 line.
I'd go for the later production models with the MIM hammer/trigger, nylon disconnector & main spring cup, wide-cut breech face & barrel tab, Novak-style rear sight, etc.
The later production guns were made as they were incrementally using better manufacturing methods and equipment. The very early 3rd gen guns used old machining, some of which involved operator control (and therefore some "more generous" variability in tolerances). More rough spots and less refinement in some of the machining, especially in the depth/shape of the extractor spring hole and some relief cuts (which would have small bits of metal break & flake off under recoil over time in the older guns).
While some folks like the narrower slide stop lever assemblies and the manual safety assemblies (less "pads" for finger engagement), the older machined hammers could have surfaces which varied from decently smooth to something approaching the roughness of a shale outcropping. This could be pretty noticeable during the DA trigger stroke as the sear nose bumped along the rough surface of the hammer. The MIM hammers were uniformly smoother, and made most newer production guns almost feel like they'd received a "trigger job" in that respect.
The MIM triggers were also a bit narrower than the previous flash-chromed machined triggers. I actually found this characteristic to lend itself to better DA trigger control. Also, the top "ears" of the trigger, which rest in the drawbar head's V-notch, tend to exhibit more uniformity in the MIM parts than the older machined triggers.
Not only did the late production 69XX's benefit from improved machining of the frames, slides & barrels, but the small parts & assemblies were continually revised and refined, as well. The newer drawbars were a lot smoother, and spots which were formerly sharply angled, or thin, received radiused corners and were strengthened in newer drawbars.
The newer guns also received the revised ejectors & extractors. The ejectors received longer tips (for faster ejection, especially with the hooter rounds used by some LE agencies) and the sharply angled corner under the back of the tip was replaced with a more curved angle, helping reduce the potential for a stress riser (and tips breaking off). The extractors received an improved hook design.
If I were going to be looking to buy a used 6906 (or 6904), and I had my druthers, it would be a later production version.
Naturally, although the 69XX magazine had 12-rd capacity (except in those half dozen states with mag capacity restrictions), the M&P 9c is still going to feel slimmer and more ergonomic than the 69XX's, and still retain a 12-rd capacity mag.
Don't get me wrong. I used a couple of 6906's from the earliest production vintage for several years, running a few tens of thousands of rounds through them (and replacing some assorted parts, assemblies & springs, of course). They worked fine for my needs. I maintained them well and replaced some older parts with newer parts, since I could do so as an armorer, and I trusted them for my plainclothes and instructor duties.
I'd not turn down a chance to pick up a LNIB old-style 6906 for the right price (since I'd go through it, anyway, replacing some of the parts and checking for normal fit/tolerances & functioning) ... but everything being equal, I'd rather have a newer version.
Just me and my own thoughts.
