Handguns tend to be a deeply personal thing. We all have different priorities and different levels of training and experience, so it can be difficult to talk about handgun selection online without talking past one another.
I have been seriously training in defensive handgunning for over 20 years. I have been a certified LE firearms instructor in my state. I average at least one professional training class per year. I teach the NRA classes to new people. At one time I held an IDPA expert classification.
There are people who are much faster shooters than me. There are people who are more knowledgable and people who are better instructors. You may be one of them. I don't know, so take this for what it is worth.
I am assuming the reason you want a 10mm handgun is for personal protection in the outdoors. If you want it for some other purpose, then this post won't mean much to you.
In my view, and in the opinion of every trainer I have ever sat under, the overwhelmingly most important quality of a defensive handgun is reliability. If the gun is not 100% reliable, then you should sell or trade it away. That trumps all else.
Once the field is narrowed to guns that run reliably, the next things are probably sights and a manageable trigger. I frequently hear people referring to a handgun having a "good" trigger and what they mean is the lightest, most crisp trigger possible. I don't regard that as optimal for a defensive handgun. You need a certain margin for safety while still keeping it manageable. I like the H&K LEM trigger a lot as a balance. Glock triggers and some of the other strikers like the VP9 are good too.
Sights are important too, but they an be easily changed on most guns these days. I like Ameriglo sights on Glocks and I use Trijicon HD's on my H&K's.
That leaves ergonomics. With all else being equal, I will consider how the pistol fits my hand and how the controls are laid out. I find that many people who are less experienced will want to choose a handgun based on "how it feels in the hand." This is intuitive, and it seems like a way to differentiate between guns if you don't know another way to do so, but it is not very useful. I find that evaluating gun fit and ergonomics is very hard to do without firing the handgun in question. You can learn a lot under recoil. I personally find that bigger and blockier guns with more surface to hang onto are easier to shoot faster and accurately than some of the more svelte models. For example I can run an H&K USP 45 faster than I can an HK45, all else being equal.
All that said, I am of the view that the Glock 20 is the best 10mm auto on the market, and the new M&P offering is not going to change that. The G20 has a very well deserved reputation for being soft shooting and ultra-reliable. It just runs. It has been around for a long time. Mags, holsters, and sights are readily available. Did I mention that it is dead nuts reliable?
The M&P line has a mixed reputation for quality and reliability. It does not really interest me in the least. If you happen to get one that runs and is not subject to any recalls or other upgrades, you will likely find holsters hard to come by. And spare parts should you need them.
Some people like the Sig 220 single action version in 10mm. I have handled but not shot one. It is much heavier than a G20. And it cost a lot more. So if you really have to have a 10mm that is not a Glock, maybe try one.
A couple of years ago, I decided that I just had to have a Colt Delta Elite. I bought one NIB at the LGS. I put about 700 rounds through it. All sorts of ammo. Tried different mags. It never once went a whole box of 50 without a malfunction. So I sold it.
I also have a Ruger GP100 with a 3" barrel chambered in 10mm. It is OK. It runs. But it recoils significantly hard (and thus allows slower followup shots) than my G20. And it only has a 3" tube, so it produced less velocity.
When I go in the woods and want to carry a 10mm, I grab the G20. Every time.
But if you want the M&P, go ahead and get one and report back.