I have both. they serve different purposes for me. I keep the photon on my keychain for emergencies. I keep the ARC in my change pocket connected to a small swiss army knife. The ARC gets the most use. It's a pain to pull out my big set of keys everytime I need a light.
The ARC is more durable, waterproof, has better battery life, and uses cheaper batteries that are easily found around the house and easy to change. The Photon is much smaller and lighter, it disappears on the keychain and adds no extra bulk, and mine has a perfectly smooth beam vs my artifacty ARC. I also have a turquoise ARC, which I like best of all. I like them both. You should have both too!