My micro filter knowledge is a little dated but most of it still holds true...

Katadyn is the brand ofchoice because they have been slowly buying up other brands like PUR. This left Katadyn, MSR and SweetWater as the major players.

Filters themselves-
First off NO filter filters viruses. PUR used to sell the Explorer series that leached iodine to kill the buggers but those have been discontinued when they where bought over. You have to chemicly treat water or boil.

It used to be that Ceramics where cheaper, had a slower flow rate, and is silted up in the field were basicly toast. Fibers were almost twice as expensive as ceramics but could be "brushed" if silted up and flowed a little faster. (that info is few years out of date and things might have changed)

Carbon- This is usually post filtering using active carbon to remove chemicals and bad tastes.

The capcity is how many gallons or liter you can get out of 1 filter.

Gairdia is teh boogeyman because its all over the place.
http://www.cdc.gov/ncidod/dpd/parasites/giardiasis/factsht_giardia.htm
"During the past 2 decades, Giardiainfection has become recognized as one of the most common causes of waterborne disease (found in both drinking and recreational water) in humans in the United States . Giardia are found worldwide and within every region of the United States."

To make things worse

"What are the symptoms of giardiasis?
Giardia infection can cause a variety of intestinal symptoms, which include

Diarrhea
Gas or flatulence
Greasy stools that tend to float
Stomach cramps
Upset stomach or nausea.
These symptoms may lead to weight loss and dehydration. Some people with giardiasis have no symptoms at all.



How long after infection do symptoms appear?
Symptoms of giardiasis normally begin 1 to 2 weeks (average 7 days) after becoming infected.



How long will symptoms last?
In otherwise healthy persons, symptoms of giardiasis may last 2 to 6 weeks. Occasionally, symptoms last longer."

Here comes my opinion.
I would get the Guide series over the Hiker line.
Guide-Wt 14 oz. Ht 9.5 in.
Hiker-Wt 11 oz. Ht 6.5 in.
Its only 3 oz and 3" taller but pumps faster and the T handle is alot easier to use.

I also recommend you use a coffee filter over the inlet acorn. It will help cut down on the silt your filter sucks up and will increase your filter's life.

Hope that helps a little.