Bucky's bionic arm is clearly an improvement on his human arm. From the page on Bucky's bionic arm from Marvel Cinematic Unvierse wiki;
The bionic arm gives him strength superior to that of Captain America. The Winter Soldier was able to rip open a S.H.I.E.L.D. armor SUV, rip apart Falcon's EXO-7 suit, and smash concrete by punching it. With his bionic arm supporting his already-enhanced natural strength, the Winter Soldier was able to overpower Captain America in a fight.
Why did they just not replace both of his arms with bionic arms, given that it's so much better than his actual arm?