I've got both on the same ring. The Arc is great for extended use since it's bright and the batteries are easy to obtain. You can also use it to drain out all the AAA's laying around. The LE version with the T3 HA is very scratch-resistant.
I actually use the Photon 3 more often because I don't always need a bright light, and it has the momemtary switch. Also, since I haven't greased up the Arc's o-ring, the photon is easier to use one-handed. I have the covert version that has a shroud around the circumference of the LED. I found that the photon 3's switch requires more care in use than the photon 2 since the front of the switch (button) is the momemtary bright, while the back of the switch is the contant mode (in my case, medium intensity or low intensity). A good percentage of the time, I've hit the momemtary switch when I've meant to activate the constant switch. I also don't find the 3 flashing modes to be much use other than the gadget factor. A red photon 3 might be more handy for the flashing.
I would recommend both, but get the Arc first. You won't be disappointed.
gm