The lithium is not really brighter in the zebralight over alkalines, but it does prevent it from going dimmer sooner, here is a graph with duracell alkaline.
I agree that the zebralight is brighter and runs longer.
However the reason I prefer the ARC for the psk are several reasons
1. It is smaller and it fits in most psk
2. It uses different regulation than the zebralight, it allows you to take advantage of dead or poor quality batteries and still have light.
3. The Durability has been proven time and time again. Do not get me wrong the zebralight is very well made but it is still relatively new. The zebralight would probably hold up very well, but I know the ARC is the least likely to fail.