What is more important: EPA or DHA ?

We need both EPA as well as DHA!
Research shows that a lack of EPA weakens the body's immune system to fight diseases and a lack of both omega-3 fatty acids (EPA and DHA) endangers one's mental health. This manifests itself in difficulty concentrating, learning problems, poor memory and it leads to an increased risk of developing depression, suicide attempts and ADHD. You should keep in mind however that low levels of EPA and DHA are not the only determining factors responsible for the creation of these disorders typical of our civilization, but they may be a very important factor.

DHA and EPA are essential for optimal brain functioning. DHA is an important building block of the brain and is crucial for the healthy development and growth of the brain. The brain of an unborn child goes through a growth spurt during the third trimester of pregnancy, and the first three months after birth. It is crucial for the subsequent brain development of the baby to consume sufficient amount of DHA to promote a healthy brain cell membranes. Later on in life, sufficient levels of DHA are necessary to keep cell membranes flexible.