Thats an interesting question, Kane. Generally, I think water filters are fine for North America. But there are indications that purifiers are in order, especially in areas that get a lot of human use.
The HyperFlow Microfilter
That doesnt mean you have to do a lot of fancy stuff. Try MSR's newish, easy-to-use HyperFlow Microfilter ($100), which lets you filter more than two quarts a minute yet still catch all those nasty cryptosporidia and giardia thingies. Or try the Katadyn Hiker Pro ($80), a proven design thats easy to use, very effective, and can be overhauled in the field as needed.
Either of those would be fine for most hikes. I have no idea where the line is, but on certain hikeslots of hikers around, fairly still water, would be my triggeryou can do some after-the-fact purifying. MSRs Sweetwater Purifier ($10/two fluid ounces) is a chlorine-based solution that is added to filtered water. It does have some impact on water flavor, but it can be hidden with powered Gatorade or other mixes. And it isnt really that bad.
Anyway, water purifying is largely a two-step process, so its no big deal to take the Sweetwater solution along as well as a filter. The exception is yet another MSR product, the MSR MIOX ($140). It uses an electrical charge (it requires batteries) and salt to make a solution that kills any bad stuff in water. It is not instantcryptosporidia requires four hours of time before its official dead. Which means that you will ALWAYS wait four hours if you want to be sure." But the MIOX is compact, easy to use, and great for filtering large quantities of water, so there is that.