If you are going to EDC, the Shield. If you just want something in the house and to shoot on occasion, the full size M&P feels lighter to me than the Ruger.
40 cal isn't much more for the ammo, and it has more stopping power.
The .40 makes a larger hole, but not by much(.40 vs .35). It has a bigger meplat as most .40's run a flat nose in practice ammo compared to the 9mm rounded nose. The slight difference in caliber isn't enough to justify ammo cost, recoil and accuracy(recoil plays a big part in a .40's accuracy). In most cases, when you compare the same HP ammo, the 9mm is the same size as the .40 if not slightly larger when expanded(varies by brand). As for "stopping power", KE is close enough that it shouldn't matter(see example below). I would take lower recoil, more magazine capacity and ammo cost into account over "stopping power". If you do not accurately put the bullet where it needs to be, it doesn't matter if it is a .500 S&W or a .22LR. You might shoot a .40 and like it, but try multiple shots in a hurry and you start walking up the target, where as a 9mm will generally stay in the target area. There is a reason why the majority of police dept's and alphabet agencies that once shot .40, have switched back to 9mm. Ammo cost was part of it, accuracy was the majority. I don't shoot +P or +P+ out of my firearms as most companies don't recommend it. Some will tell you the pistol will take it but wear and tear is faster so I avoid it. I have been carrying Speer Gold Dot in either 124r or 147 gr since I bought my first pistol and haven't seen any reason to change. I have seen a lot of new ammo that REALLY makes me want to change, but I haven't had a reason to yet. If I did change, I would choose Federal 124gr HST.
Example- Speer Gold Dot- The KE(stopping power) is from Speer's website, the expansion is from FBI tests. I used Speer Gold Dot because it has been an industry standard in performance when it comes to pistol ammo. It is the one every other ammo is compared to in tests.
- 9mm 124gr +P Gold Dot expansion- .68"- KE- 410 ft/lb
- .40 S&W 165gr Gold Dot expansion- .65" - KE- 484 ft/lb
74 ft/lb of kinetic energy isn't enough to make a difference in stopping someone. Most of the time, the bullet is going to penetrate and come out the other side, so most of the energy is lost anyway.
Here are some things I have learned that came out of a class by an ER doctor. I can't remember which hospital, but he has seen/treated more gunshots than the average ER doc.....
There is a Dr, nurse, police officer, fireman, emt/paramedic, or someone that knows first aid on just about every street corner today. Emergency response times have increased dramatically over the last 20 years and especially since 9/11. This has brought gun shot survivability to 80%. If you take suicide out of the equation that number increases to 90%. The biggest contributing factor to someone dying by a firearm is shot placement and not caliber. If the bullet isn't placed in center mass(heart/lungs) or the cranium, there is a great chance(90%) that the person is going to survive because of aid received directly after the shooting and before the person gets to the hospital.