Los Angeles is a desert. Still, water flows through the mountains, even though sometimes it's just a trickle.
If you plan on backpacking through the Angeles or the San Bernardinos, the heat and the altitude will have you dehydrated with the quickness. And though various streams and creeks descend from the summits, drinking right out of the current is just asking for giardia. Or worse.
Enter Katadyn's Hiker Pro. At less than a pound it's a no-brainer to have in your pack at all times, especially when hiking in arid, heavily-traveled Southern California. The beefy filter and filter covers ensure that you'll get clean water for a long time without a filter change (unless you're hiking the PCT).
Iodine is no doubt lighter but can leave water tasting weird and its effectiveness is limited. The SteriPen may be smaller but they can be glitchy and considering all the contaminants in SoCal streams, UV light is hardly a reliable purifier. Other pump purifiers can be awkward and take forever to get that sweet life liquid flowing into your water bottle. When you're really cranking, it can fill a Nalgene in 30 seconds or so.
The intake works in raging rivers (like the Lower Kern, just to name one), trickling streams (one time this came in clutch while bushwhacking through an overgrown canyon on a 100 degree day hiking Mt. Lukens) or even snowmelt (Mt. Whitney comes to mind).
The best gear has fewer components that could cause you trouble and none of those components are too complicated. That's the problem with the SteriPen, if the electronics burn out and it's all you brought, you better hope you find a magical mountain spring somewhere. Because the creeks and rivers within a few hours of L.A. are filled with so much human, animal and industrial waste that you're taking a serious risk sipping it straight.
The Hiker Pro is a key stowaway for any long day hikes or overnight backpack camping trips into the scraggy mountains that hem in Los Angeles. Just throw it into your pack, forget about it until that big hard desert sun dehydrates you like the fiend it can. Then head back to that trickle of water you passed half a mile ago and quench your thirst.