An RO/DI filter is better. The ion exchange filter is the DI portion of a RO/DI filter. In RO you pass the water through a very fine mesh membrane which removes very small particles from the water.
www.airwaterice.com Products and answers for your water treatment needs. sells a nice little RO/DI filter for $99 called the MightyMite. A number of us have it. I've also heard good reports on I believe the Filter Guys but I'm not sure of their website.