It really depends on your liking and what you're looking for in an hand held device. by specs note 8 pro is superior than a51 (mostly). But a51 has super amoled and some cool ui2 features. you should also consider vivo s1 pro and compare these 3 phone. you can't have every feature in a phone so, do opt out those which you wont use.
i am personally using a51 for few weeks, its a good phone and again it all depend on my need.