Is "persuasive techniques" or "persuasion techniques" more grammatically correct?

I would attach different connotations:

Persuasion Techniques refers to methods used to persuade or bring someone to your point of view.

"The senator used various persuasion techniques to convince her colleagues to vote for the bill."

As mentioned, Wikipedia calls these different modes of persuasion and puts them in the categories: ethos (presenter credibility), pathos (emotional appeal), logos (logical appeal), and kairos(time and place).

Persuasive Techniques puts the emphasis on techniques, specifically the effectiveness of these techniques.

"Waterboarding is a persuasive technique for extracting information."

Both terms are equally correct grammatically. Persuasion techniques is an attributive noun (persuasion) modifying a noun (techniques). Persuasive techniques is an adjective (persuasive) modifying a noun (techniques).

But is it idiomatic to modify techniques with an attributive noun, rather than an adjective? Sure. Take a word that has no easily available adjectival inflection, say, dance.

Dance technique is considerably more common than either persuasion technique or persuasive technique (ngram).

Perhaps that's not a good comparison, because we have no choice in the case of dance. Then let's make a quick hop to ballet and see that ballet technique is far more common than balletic technique (ngram).

However, this pattern is not consistent. A further leap, but still in the realm of graceful movement, surgery technique barely registers when compared with surgical technique (ngram).

Interestingly, this example introduces a new and relevant wrinkle. Surgical can mean either "relating to or used in surgery" or "denoting something done with great precision." This means that some of the examples of surgical technique found in that ngram probably don't describe techniques used for surgery. They could describe techniques for doing something entirely different, but which are performed with surgical precision.

This may also be the case with persuasive techniques, which might describe a technique that is itself convincing. Theoretically, this makes the phrase persuasive techniques ambiguous - we might not be sure whether it describes a technique for persuading, or a technique that persuades. A phrase that's ambiguous is not wrong, but it often makes sense to avoid ambiguity where possible. That said, I'm a little skeptical that realistic contexts exist where the meaning of persuasive techniques wouldn't be obvious. And if the phrase is only theoretically ambiguous, not actually ambiguous, there's probably no good reason to avoid it.