NutraFlax just doesn't have anything extra in it. That's why I use it. I feed a very complete vitamin/mineral supplement, so I'd rather not have any extras that my horse might not need, or might be too much of something.
I have horsetech.com add some extra amino acids and biotin to it though, to help my mare grow better feet. I love horsetech.com for their customization. They're awesome to work with. Once my vitamins run out, I'm considering having them mix the NutraFlax with their new all-around vit/min supplement, so I just have one bucket for both.
Simplifly is for flies. It kills fly larvae that's laid in the manure. It works GREAT! We hardly ever have any stable or face flies and it doesn't hurt the horses any. We start feeding it in March and keep with it until November (we have a LONG fly season). My horses have been getting it for 5 years now with great results. SimpliFly with LarvaStop Fly Growth Regulator
You do need to have ALL the horses on the property get it though. It won't work if you board. Your BO will have to agree to buy it for all the horses.