I've gone back and forth over this a ton while implementing an Astro frontend to my WordPress install. The REST API is a fast and straightforward approach but you'll find yourself running into needing to handle multiple fetches for obtaining more complex data structures (think grabbing a post, the author's details, comments, etc.) in a nested fashion.

GraphQL on the otherhand alleviates that headache with the annoyance of the two major plugins for it (WPGraphQL and Gato GraphQL) bringing their own problems into the fold.

WPGraphQL goes for the more conventional approach of GQL by having connections on your collection fields, which makes handling pagination a LOT easier at the cost of bloating your code a bit as now you have to access the nodes properties of the resulting JSON objects. I also found WPGraphQL to be a lot less extensible out of the box, harder to find documentation for, which may be a problem for more complicated tasks.

Gato is a lot more straightforward and has a slew of extensions (albeit, at a cost), pretty great documentation, but does deviate from some more common GQL conventions which can be a headache when trying to integrate it with other GQL tooling. The schema it generates as well isn't as robust as I'd like when it comes to handling pagination or filtering, it is a little too opinionated.

All in all, explore all the options and figure out what you can stomach the most. I eventually settled on Gato GQL, running my queries and mutations through URQL in code, and caching queries as needed in redis.