Also, HEDP is more chlorine resistant while EDTA breaks down from chlorine so quickly that you will notice an increased chlorine demand from using it and need to add it much more frequently.
The more recent Natural Chemistry Metal Free appears to have citric acid as a reducing agent (similar to our recommendation of ascorbic acid), but the metal sequestrant appears to be some sort of anionic (negatively charged) polymer. They note that you can't use that with normal clarifiers in the water (they are normally positively charged polymers), but it's possible that the Metal Free product is effective and resistant to breakdown from chlorine -- we just don't know and haven't heard enough experiences from users to make an evaluation. So for now, HEDP-based products are the recommended metal sequestrant.