Saya is more formal than Aku. Saya is used in formal settings, for example in an academic seminar or to older person. Aku is used in informal settings or to friends or close friends. Your close friends may differ in age,but if he/she is close to you, you mas use "Aku".
But sometimes people feel that "Aku" creates distance. Some other people feel that "Saya" creates distance. But as long as your friend is okay with whatever you use, so be it.
I suggest that you use the first paragraph for simplicity in use.