That's really a difference of particles. ~는/은 is used for the topic. ~가/이 is used for the subject of the topic. In this sentence, you do not need a topic because it's so simple.
When you want to talk about your thoughts on something, you can use both. (I think) this food is really expensive -> 저는 (topic = according to me) 이 음식이 (subject = this food) 진짜 비싸요.
If you want to see 저는 and 제가 together (which would be a harder sentence), you'd have to talk about yourself. 저는 제가 그 남자보다 더 강한 것 같아요 -> It seems to me that I'm stronger than that guy.