PD: I have read that both are good although it also said that after
washing the flourite so it won't make your tank
cloudy you might as well be back to aquarium gravel. Is this true?

I don't know where you heard this, but I can tell you this is not the
case. As far as which to use, Onyx or Flourite, it is up to you. Both
will be great for plants. The Flourite does contain higher iron
levels, but if you dose iron, this will not matter.