That is an EXTREMELY complicated question. Even for someone with a degree in chemistry, the discussion regarding iron choice is a confusing one.
The iron plants want, Iron II, is short lived in water and will oxidize to Iron III if not protected. To slow this down, you use a second molecule to sequester it from oxidizing agents. We call this molecule a chelator. There are three different chelators available in aquaria: Iron Gluconate, EDTA Iron, ad DTPA Iron. Flourish Iron is gluconate, CSM+B uses EDTA, and DTPA is usually bought by itself. They very in strength by the order I introduced them. Gluconate is weak chelator but is easily digestible by the plant (true, and this is how seachem promotes it, but I doubt that it's that big of a deal). EDTA will hold iron for longer, but usually only a matter of hours. DTPA will probably hold iron for a few days or more.
Which chelator is best for you will largely be determined by your KH and pH, with high KH/pH needing stronger chelators. Honestly, if you're planning on buying something for iron issues, DTPA will be the way to go.