Waterfall using faux stones made of styrofoam. Use Drylok or Grout?

[LarryMac]

I'm planning a waterfall feature for my terrarium (maybe future vivarium). The build would start with a rocky hillside area made of Styrofoam with a tube thru it for the little pump. I've seen folks use Drylok and others use Grout to make the sculpted foam look like rock. Which do you believe would be better? I will also likely cover it with epoxy resin but not sure about that.