I'm hedging my bets and alternae using both!
The ferrous version (Flourish) claims that plants can more easily uptake iron in this state, which sounds good to me. This version does however, cloud my water for awhile.
I've had good results with ferric versions as well, with no clouding. Plus it has added manganese. I can't see a difference in my setup, so unfortunately I don't have a definitive answer. In theory, the ferrous version should be better.