Hello,
This is one of those situations where we make life more complicated for ourselves with very little to show for it.
Iron is a micronutrient, which means the plant only needs microscopic amounts of iron. About one hour after you have dosed iron, the plant has already taken up it's fill. Of course it's easy to pummel the plant with more iron than it needs without any ill effects, but why bother? Why spend more money and effort for specialized chelation when it is already overkill with the less efficient chelated methods? I really doubt you will see any benefit.
A lot of this hand wringing over [insert your favorite chelator here] arises because for generations, people have misdiagnosed the basic deficiency of plants. They see yellowing of leaves and immediately assume it's due to lack of iron. There are even folks who spend their hard earned cash on [gasp] Iron Test Kits. In 99% of these cases, their tap water already had sufficient iron and the real deficiency is Nitrogen, but since everyone becomes hysterical at the mere thought of adding Nitrate, then one myth feeds another to create pandemonium.
My advice is to use the cheapest and/or the most convenient source of iron and simply carry on. If iron is present in your basic micronutrient mix, then just use that without worry.
Cheers,