Quality of back pannel, look, In hand feeling is different, previously I used A50 now I'm using M51 i found a difference in these two and M series back pannels having simple design compares to A series phones
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.
main difference is one ui features, and optimisation A series run on full fledge of one ui and very well optimised inspite of same processor your experience will be faaar better than M series and M series mainly famous for bigger battery but not much difference in battery back up considering big battery in M series but also all A series is not good, A50s or higher model is better