The type I bought was one that you don't have to hook up under the sink and can take anywhere. I didn't buy the tank because I have two aquariums that I have to do water changes on so I basically use a bucket from Home Depot with a hole in the top, put the line with purified water in the hole and let the other drain the dirty water out. Then I dump it into a 30 gallon trash can I use to mix my saltwater.
The unit you get depends on how quickly you want it to make water (gallons per hour) and whether you want to put it under a sink or not. I would go with a deionized unit as well.
BRS has good quality stuff as well.