Research and testing credentials were the main factors for me to considering this particular brand.
I extensively researched to see which system would have the evidence to back up their claims of 99% removal , including removing specifically chemical runoff , heavy metals etc.(as I often source water near farmland)
This system came up with the testing done in 4 different countries including , importantly for me , testing here in the UK by the national laboratory and the environment agency.
Sales to the Red Cross and peace keeping forces worldwide also helped trust the system .
A few other systems did have good credentials but either failed to remove certain chemical contaminants and trace metals or ,did all those things, but fell short on the price of each replacement cartridge filter .
400 gallons of purified water for less than £20 is a good deal , believe me .
Also the automatic shut-off valve system, when filter is no longer efficient was a good selling point for me as a foolproof method for getting it right. Plus with kids I didn't want to take chances.
More information about the tests and procedure on their front page and here:
http://drinksafe-systems.co.uk/worldwide-testing.php