It's going to depend on the water they start out with. It's hard to say, because there's not really a standard of purity in the end product. Each are made differently and treat the water in different ways that excel at removing different contaminants. Here in the U.S., most tap water is filtered through a reverse osmosis system at the water treatment facility. So it basically is tap water for many of us, or perhaps slightly better because it doesn't have to run through a gauntlet of pipes before reaching our house. For photography chemicals, deionized would probably be better, as it removes more metals from the water (though not many bacteria or viruses). For really pure water, both steps are often used, as neither one does it all.
Also here in the states, we can buy distilled water fairly cheaply in gallon jugs at grocery stores. That's what I use. It's not 100% pure by any means, but it's a good, cheap solution that works for photographic purposes.