should I use Buttercream or Royal Icing?
I think either would work. Royal is harden, so if you want to paint on it,
this is the good. Buttercream, make sure it isn't too stiff.
Personally, I've used royal.
You could use either one. I haven't tried royal icing on fondant..but I was happy with the buttercream! Goodluck!
Either one will work fine. I usually use RI on my cakes because it dries hard and I don't have to worry about it being damaged if I bump into it or something. If you have a lot of piping, lots of grass or something, bc might be better since it's softer, better when you eat it.
Good luck on your cake!