Eelektross easily. While Klang has access to moves such as Shift Gear and Gear Grind when set up correctly can turn the wonderful steel-type into a powerhouse, Eelektross easily beats it in a various range of things, with access to moves like Drain Punch and the other variety of punches, a huge diverse movepool, Eelektross can take down a huge range of Pokemon in the NU tier. If Klang and Eelektross had a battle it would be most likely Eelektross would win with a few Drain Punches to restore it's lost health while also chipping away with Fire Punch.
I also adore Eelektross's design, a huge eel is fascinating and awe-striking. Klang for me was like the gear form of Magneton but then again there are other great Pokemon which also follow along the same lines. But yeah in-game wise Eelektross is still better, I've always found it tricky working with steel-types in-game at least but that might just be me! Overall my vote goes to the wonderful electric eel, for better everything. :>