They're very different characters, honestly. IX is most definitely Garnet's story while you could argue X was justas much Yuna's as Tidus'. I mean, did Garnet ever have a super awesome moment like Yuna did with Yunalesca? It wasn't Tidus saying "no, screw you and your traditions!" It was Yuna.

Garnet really never had anything like that. The one time she tried to mouth off to the villain, Kuja just laughed in her face.

Overall, Garnet is handled like a delicate flower while Yuna is treated much more respectfully.