I think there is something more important to consider than whether you are better at maths or Japanese.
Rather than thinking about which you are better at, thinking about what you actually want to study is more important.
「どちらの方が得意かよりも、何を勉強したいのかの方が、文理決めるときに大事ですよね。」は英語で「Rather than thinking about which you are better at, thinking about what you actually want to study is more important.」
その英文章を言う際に「want」(~したい)という単語をトーンで強調した方が良いと思います。
I think there is something more important to consider than whether you are better at maths or Japanese.
(国語と数学どっちの方が得意かよりも大事なことがあると思います。)
It is important to think carefully about what you actually want to study.
(自分が何を勉強したいのかちゃんと考えた方が良いと思います。)
ボキャブラリー
rather than ~ = ~よりも
think about, consider = 考える
important = 重要、大事、大切
better at = ~のようが得意・上手
ところで、「理系」は英語で「science」や「science subjects」といいます。「文系」は「arts」や「humanities」といいます。
ご参考になれば幸いです^^