The main difference is is in the software. A series run on "One UI" and M series run on "One UI Core". The difference between these two is that you will not get some features like Samsung customization apps in One UI Core. Another difference is in battery. If you don't care about features, then you should go with M series because of their massive 6000, 7000 mAh batteries. I personally use M31s because I care only for battery and not for features. So choose wisely. And there is no huge difference in price, only a small difference in price is there.