Iron doesn't dissolve easily in water so most Iron fertilizers chelated iron. The most common Chelates are EDTA DPTA and occasionally you see EDDHA. Seachem uses iron Gluconate.
EDTA is stable at a PH of 6.5 or lower. If you tank PH is higher than this EDTA will seperate from the iron and plants will not be able to use it. DPTA is stable to about a PH of 7.5. EDDHA is stable to about a PH of 11. Unfortunately EDDHA is the most expensive. Gluconate is apparently not very stable but it is more easily absorbed by plants.
So which one will work best for you will depend on your tank PH. You can use a fertilizer calculator such as rotalabutterfly.com to determine how much to add to your aquarium.