Which mantra should I chant for a peaceful life? Hare Krishna mantra or Gayatri mantra?
—Rajender Kalra, India
The mantra you choose is determined primarily by its effects upon your consciousness. The Gayatri is a mantra for both personal and global enlightenment. It is therefore by definition a prayer for peace. The Mahamantra (Krishna) is a beautiful and powerful chant for the realization of God, whether in the form of Krishna or in the impersonal form including as peace, joy, love etc. So truly either one is wonderful.
You might experiment over, say, a week with alternating these and see what you experience and what you feel most in tune with. If your devotion is more personal then the Mahamantra may be best; if your orientation is more impersonal and wants to include all humanity than the Gayatri is great. The Gayatri is among the oldest and most sacred mantras, too. It is considered the seed of all mantras and derives from the Rig Veda.
Blessings to you!