I've used both, and between those two...
It does't matter. They're both close enough functionally for you to make a choice based on personal preference. I like the Pulsar II a little better.
I both agree and disagree with the others here: I agree that there are better options; I disagree that the ARC AAA is the one you should choose.
For a few dollars more (I think they're $17, but I could be misremembering a bit), you can get an Avexa Free Light. It's about 1.5-2x the size of the Pulsar II (it's the length of two quaters in a row, and the thickness of four quarters). It has no constant-on switch, but it is easy to add one yourself. If you can settle for these minor disadvantages, you get:
1) solar powered. This means you'll never pay for batteries for the light, or have to change the batteries, again. It also means that you are more likely to have a full charge when you need it. And since it is usually kept fully charged, it usually has the maximum brightness that comes with new batteries.
2) really, truly waterproof
3) surprisingly tough and shock resistant
--Mike