5mm LED lights including the coin lights and the Arc AAA are all pretty much the same in terms of brightness. The difference is in size, construction quality, etc. The Arc AAA uses a common, easy to change battery and is tough as heck (hard anodized aluminum) but it's kind of a large light and was expensive even when it was still being made. Now that they're scarce and sought after, prices are through the roof.

The Photon II is about the smallest and most lightweight of the CR2016-powered lights, and it has a constant-on switch which is missing from some lights. Its main weakness is not-so-good water resistance, plus you need tools (tiny Philips screwdriver) to change the batteries. However, it's still the classic example of this type of light. The Inova Microlight (version with constant-on switch) is a bit bigger and uglier, but perhaps an improvement. It's more water resistant and you can change the batteries without tools.

The best deal in these things is the Countycomm $1.00 lights (they come in bags of 20):

http://countycomm.com/light10.htm

It's worth buying a bag of 20 of them since you'll find yourself constantly handing them out as gifts. They're always appreciated. They are like the P2 in needing tools to change the batteries and in not being so water resistant, but they're quite easy to operate. Their main extra weakness is they can turn on by accident if you drop them or slam them against something. If you put one in a PSK, I'd advise putting a piece of scotch tape over the switch to keep it from turning on by accident.

These small lights are amazingly handy. They do get dimmer as you use them, but they'll run for several hours before getting too feeble. They start out comparable to a typical old-fashioned penlight. Even after several hours, they're more than bright enough to find your way around a dark room or stairwell. If you've ever been caught in a lightless situation you'll know how important that can be.