The main factor to consider is that the M5906 is more complex to use because it has a heavy first-shot double-action trigger pull and a decocker/safety. The M5906 has a positive trigger-actuated firing pin block, so it's totally drop-safe; many S&W DA/SA shooters carry these guns in DA/safety-off mode and only ever use the lever as a decocker so they don't have to worry about accidentally leaving the safety on. That said, you WILL have to decock the pistol from time to time, and you WILL need to learn two trigger pulls.
If you intend to CCW the gun, the M&P is slightly smaller and a
lot lighter. The listed empty weight for the M&P full-size is 24oz vs. 38oz for the M5906; IOW the M&P weighs about the same amount
loaded with 17rds of 9mm that the M5906 weighs
empty. If you intend to CCW the gun, I recommend bypassing the M5906 in favor of the alloy-framed M5903 or M5904, or better yet, one of the S&W single-stacks (39xx series) or alloy-frame double-stack compacts (69xx series).* FWIW I have a M3904, an alloy-frame single-stack full-size; it weighs 27oz empty and 34oz loaded, a lot less than the M5906 and slightly less than the M&P, but at the cost of nearly half the capacity (9+1 vs. 15+1 or 17+1).
At the range, the extra weight of the M5906 will soak up a lot of recoil. However, S&W DA/SA pistols have a very high bore axis, i.e. the barrel centerline is relatively high above the grip. This, together with the stiff metal frame rather than the soft polymer frame, makes for more felt recoil and muzzle flip for the same weight pistol. This is more apparent with the lighter alloy-frame DA/SA variants (see above) but is still a factor with the heavier ones. IOW the M5906 should still shoot a lot softer than the M&P40, but the difference may not be as drastic as you think.
Both guns have good aftermarket support. The M5906 has an advantage in the spare mag department because of the widespread availability of used mags; the M&P has not been in circulation long enough for many unwanted mags to be floating around. I expect the M&P to eventually eclipse the 59-series in this respect, but it will probably take a while.
Couple extra notes re: the M5906 aftermarket...
- The 3rd-generation 59-series, which includes the M5906, came with three types of rear sights. The standard rear sight for the first few of years of production was a vertical drift-adjustable rear blade, which was later superseded by a ramped Novak Lo-Mount drift-adjustable sight. A fully screw-adjustable unit was optional. The Novaks have the most aftermarket replacement options and IMHO are the best for CCW. The adjustable sight is great at the range but is protected by BIG metal wings that are not CCW-friendly. The 3 types of rear sight are NOT interchangeable because the dovetails in the slide are different sizes! (You can change the whole slide, but this is kinda like the tail wagging the dog. )
- Early 3rd-gen 59-series pistols had a squared-off trigger guard that was later rounded off. Holsters for the square trigger guard pistols will generally accommodate round trigger guard pistols but NOT the reverse. Keep this in mind if you have a favored holster design.
If it sounds like I'm dissing the M5906, this is not really my intent. I'm a big fan of the S&W metal-frame centerfire automatics. They're very high-quality guns with a solid feel that's missing from tactical tupperware like the M&P. However, they DO have a steeper learning curve than the M&P, and the M5906 is not the best choice for CCW if that may eventually be in the pistol's job description.
*I noticed you're in Ft. Worth. I know a LGS that has a whole bunch of LE trade-in M6906's, the stainless-slide / alloy-frame 12rd double-stack compacts, for cheap. PM me if you're interested. I don't work for them and they didn't pay me to write this, I'm just trying to be helpful.