First, let's compare them.
Pros: high defense, great defensive typing, solid special defense and speed.
Cons: It's a physical attacker with very few physical moves, and it's weak to common types in this game.
Pros: Great speed and solid special attack, wide move variety
Cons: Atrocious defense, merely modest HP and special defense.
Now, how do they fare against the rest of the major battles?
Melony: Klinklang is super effective against Ice, but not that Lapras. Raichu is good against Lapras.
Piers: Raichu can learn Brick Break, but it doesn't get STAB. Klinklang doesn't learn any super effective moves against Dark.
Raihan: Klinklang has no moves super effective against Raihan's Pokemon. Raichu can learn Play Rough, Brick Break, and Focus Blast to hit for super effective damage.
Champion Cup Marnie:
Klinklang's Steel moves are super effective against Grimmsnarl. Raichu's Fighting moves are good against everything but Grimmsnarl and Toxicroak.
Champion Cup Hop: Both do fairly well here, but Electric type moves from each are resisted by Rillaboom and Pincurchin.
Bede: Klinklang's STAB is super effective against all except Mawile. Raichu's STAB is all neutral.
Nessa: Raichu's STAB is super effective against everything Nessa has. However, Klinklang also gets Electric moves.
Allister: STAB of both Raichu and Klinklang is neutral.
Raihan: Raichu gets Play Rough. Super effective except for Turtonator and Torkoal. Klinklang does not get anything super effective against Raihan.
Chairman Rose: Rose's Pokemon tend to resist Klinklang's moves. Raichu gets Fighting moves still, however.
Champion Leon: Klinklang is able to hit three of his Pokemon super effectively. Raichu with STAB is super effective against two. But if Raichu has Play Rough, it can also hit Dragapult and Haxorus super effectively.
Overall, I would say Klinklang is the better choice. It can learn Electric moves, so Raichu may be unnecessary. It also has better defenses than Raichu. It doesn't have too much coverage, but it's able to deal with pesky Fairies that the rest of your team can't seem to hit super effectively.
Hope I helped!