Not sure about what works better on slugs, but as per safety, I'd consider SDS as opposed to whether someone thinks they are organic. Iron Phosphate is completely benign, though eye irritation can occur. Sodium Ferric (EDTA) is a little less benign. Not carcinogenic, but can trigger allergic reactions, and may cause respiratory damage if ingested in large quantities.

I've been using iron phosphate bait for years now and can attest to its effectiveness. I believe that one's OK for organic growing, since it breaks down into fertilizer, basically.