The honest answer
Both tools answer the same basic questions: how many people came, where they came from and which pages they read. The difference is how much more you get, and what you give up for it.
GA4 is Google's analytics product and it is free for most sites. It is event-based, has many reports and connects to other Google products. That depth has a cost. It has a learning curve, and it uses cookies and identifiers. In many regions, for example under EU rules, it needs consent before it runs.
Plausible is lightweight, open source and focused on privacy. It does not use cookies and does not collect personal data, so it is commonly run without a cookie banner. Check your own legal requirements before you rely on that. You get one simple dashboard and fewer features. It is a paid hosted service, with the option to host it yourself.
Neither tool improves rankings. Analytics shows what happens after someone arrives. It does not explain why a page is missing from search. For that, read crawling and indexing, explained simply. Every script you add also has some effect on loading, which matters for Core Web Vitals. Run the free audit after you install either one.
Features and plans change, so check each product's own site before you decide. For the cost, see the current pricing on each site.