Mainly the two filters differ in design. The Katadyn Guide Pro ($86; katadyn.com) uses a glass-fiber filtering system with a carbon core to remove off tastes. The MSR MiniWorks EX ($85; msrcorp.com) uses a ceramic/carbon filtering system that also is designed to improve water flavor. One big difference is that the Guide Pro has nearly twice the water output for the same number of pumps.
I like the MiniWorks largely because Im familiar with the design, having used it for a long time. I also like its pump action, which uses a lever that you work like an old-style water pump. Its a little more ergonomic than the piston-style pumping action employed by the Katadyn. Also, I use Nalgene bottles a lot, and I like the way they screw right into the bottom of the MiniWorks. So, yeah, I guess I think the MiniWorks is a little bit superior. But thats partially a personal preference issue. In terms of removing bacteria, protozoa, and other bad things from water, theyre very similar. Keep in mind neither deals with viruses so they are filters," not purifiers."
In your case, due to your investment in Platypus water bottles, I think it makes perfect sense to get the Katadyn filter. It will work well for you. The Ouachitas look like interesting country. Hope you have a great trip!
The Gear Guy reports from 2007 Winter Outdoor Retailer, the bi-annual gearapalooza in Salt Lake City. Check out his top picks for gear to watch in 2007.