R2D2 has existed from the beginning all the way to the end of the whole franchise outliving a lot of the protagonists.
He hacked and saved the heroes and did a lot of heroic deeds all the way to the end.
That spinning ball has hardly lifted its weight. Sure it did a thing here and there but it just a copy and a gimmick to sell the orange ball to children.
I don't know which droid serves a bigger purpose. Oh yeah BB8 wasn't introduced until Episode 7 while R2 was serving since the Phantom Menace. They both beep, Carry and transfer information, And they both work for the rebels/resistance. So in the end, DO NOT ASK ME THIS QUESTION AGAIN!