The M&P 10 is itself a great rifle. The downside is some proprietary stuff.
The sr25/lr308 seem to be what become the most common design and so it is easiest to get extras for items that share parts with those.
Even Armalite switched (they offer models in both types) to the sr25 magazines because people preferred that option over being limited the Armalite's magazines.
Fortunately the M&P 10 shares the same magazine and a lot of the furniture, trigger group so all drop in triggers will work, etc but some things are different.
Gas tube length, bolt carrier group, etc
Apparently S&W also does not just send people some of the proprietary parts if they ask to buy them either, like a spare barrel. Which has led to people figuring out how to install DPMS and similar barrels and bolt carrier groups. They don't just drop in, and some parts have to be removed from the S&W first before they can take a regular bolt carrier group.
So the rifle itself is great, but proprietary stuff combined with a company that does not just send you any part you want to buy makes it being proprietary a pain once you have any problems or need to replace significant parts.
In an AR-15 which is more standardized that would definitely be a deal breaker. But 308s are all a little different, some are just more different.
So you have to look at it as an individual rifle with its own merits.
While the M1A is nice and classy. It also weighs more, and takes a lot more work to make accurate. How you bed the rifle makes a big difference. With the S&W it will be more accurate from the start with 168s even without a free floated barrel and you don't have to worry about bedding and rebedding the rifle. M1As require a lot of tinkering to get the most out of them, and just taking the action out of many stocks undoes some things. The AR will be the same unless you actually change something significant and you can reasonably disassemble it all the time and it maintains the same zero.
Optics are also very easy to use with the AR platform.
One thing I do prefer on the M1A is the lack of a buffer tube or other spring or parts behind the receiver, allowing the overall length to be less if you so desire to reduce the length of the back end, or a folding stock to be used while remaining functional. Even bullpup designs can be made based on it for the same reason (though the good quality rogue is a bit heavy and they have not been making them lately.)
Stock vs stock, the AR would be my preference.