Question:
what is the difference between the two? (if any).

i go hiking periodically in Yosemite and have (in the past) borrowed a MSR Miniworks water filter... i got a SweetWater Purifier for Christmas as a gift... i don't know if i want to exchange it for the MSR or not (i'm familiar with the Miniworks and liked it), but if a "purifier" is something different and better, maybe i should stick with the sweetwater brand? or is this just different wording for the same principle? if so, i'll exchange it for the MSR, besides, the MSR has a much better review.

Answer: A 'purifier' removes all bacteria and viruses. A 'filter' removes all but the littlest of viruses. Somewhere I had a link to an EPA site that explained it more scientifically, but that's it in a nutshell.

Some folks believe they need the 'purifier' for wherever they hike. BOMK, there is little need for one in most of the US. Check with whomever manages the land upon which you intend to hike.

Personally, I too, am a huge fan of the MiniWorks. I've had mine for five (?) years, I've pumped a lot of water thru it, and I have had zero problems.

While checking out the miniworks on the web I found this statement...

PLEASE NOTE: If you live in the state of Iowa, we are unable to sell you any water purification system due to Iowa Department of Public Health Restrictions. We apologize for any inconvenience.

Purifiers remove viruses in addition to all the nasties that filters remove. Viruses generally aren't a problem in the USA unless you're taking your water from someplace like the Monongahela River. They are pretty much unheard of in the backcountry and definitely not an issue in Yosemite.