When both Pokemon are given the spread 252 Atk/4 SpD/252 Spe adamant, and Klang is given an eviolite, Klinklang's defensive stats come out to be 261 HP, 266 physical defense, and 207 special defense, and Klang's are 261 HP, 339 physical defense, and 310 special defense. Klang is obviously better at being a tank, but that leaves it outclassed by even better tanks like Steelix. Klang can be used as a shift gear sweeper, but since it has bad physical attack and speed, it would be outclassed by Klinklang. Basically, Klinklang is better than Klang because Klinklang actually has a niche, while Klang gets outclassed no matter what it does. As you can see from this statistic, Klinklang is used about 46 times as much as Klang.
Note that if you're playing a format where Metagross, Lucario, Magnezone, Cobalion, Celesteela, or Kartana are allowed, you should use one of those instead of Klinklang.