Are you considering the M&P9FS for Law Enforcement (i.e., "uniform gun"), or just concealed carry?
I've not been in uniform (semi-retired rent-a-cop, among other things) since the 70's, but even the M39 was an improvement over the 6" M19 Combat Magnum I was carrying then. I had an M10HB for concealed carry (nobody cared about printing). The M&P9FS (I've got the 40FS) is roughly the same size & form factor as my old M10, or even the M19, if you ignore the extra 2" of barrel. Easy to shoot, and accurate enough.
The 40FS is a little snappy, but it'll do what you need.... The 40FS is essentially the same gun as the 9FS. With the same advantages. Personally, I'm not excited about the 9mm over the .40, but YMMV, and it's your choice. (The 9FS should shoot just like my old M10HB - the 9mm is a tad snappier, but overall handling should be exactly the same.)
If you're looking for concealed carry (as a licensee), the 9C or 40C may be better choices. Both will kill paper about as well as their full sized counterparts, but the Compacts are just easier to carry.
SOME shooters will do better with the longer barrels, and, of course, the larger guns have larger magazine capacities, but those may be the only advantages.
(I bought the M19 because the Military was taking about every 4" M19 that S&W could make at the time for Vietnam. The 4" M10 got to me a little later, and I grabbed it because I couldn't hit much with a snubby, and needed something for concealed carry that I could trust.)
I don't carry my full sized .40 much - I'm big enough to get away with it most of the time, but the 40C is just easier. I wear that around the house most of the time, with a spare magazine from the 40FS. If I have to reload, I'm going to be putting a lot of ammunition into the handle
.... (They're interchangeable.) OR, if I did have the itch to carry the full size .40, the spare magazine on my belt fits....
("Outdoors", I rotate between a pair of Officer's-sized 1911's. Fewer rounds, but lasers and night sights. These were put together - at serious expense - before I got involved with the M&P's. The spare magazines are also a little easier to carry/conceal.)
The holster I use is a cheap "slide" type from DeSantis. It's got a top strap (thumb break), and an open bottom that allows me to use any of my M&P's in the same rig. This one has a slight forward rake, and rides fairly high. Can't think of the name, though.... I use essentially the same holster for the 1911's (although they're not interchangeable).
I think my 40FS came with a Kydex holster and maybe a mag carrier. Can't find the latter, and can't imagine buying the former (I hate Kydex
), but it's not too bad. From a concealment standpoint, considering the bigger than a breadbox gun, it's not bad, either.
(I had an XD9SC a while back that came with a Kydex holster. Unfortunately, it canted the gun out and away from your body, making concealment iffy. Not a bad rig if you're going to tuck a bomber jacket behind the holster, leaving the gun exposed, but otherwise downright silly. The mag carrier that came with it was just as bad....)
Guess my point, though, is go for it! Try one at a range if you can (actually, try both the full size and Compact, and the .40 if you can), and see if you like it.
Consider, too, the Apex DCAEK kit for your new gun. IMHO, a big help.