For writing long strings of words on a cake side?
I am thinking that royal would be best; what do you think?
AWhatever it is, it has to be stiff and fast-setting, unless you plan on tilting it up into position. I've written on the side of a naked pound cake (when it's baked in a Bundt mold, there's no top to write on!), and it was a royal pain in the butt, just getting "Happy 77th Birthday Daddy" onto the cake.
It might be a good idea to write your message in RI on a flat surface and transfer to the cake if you can? Writing on the side of the cake neatly I found to be really hard.
This is quite tricky - for a wedding cake with poetry written on the sides all around the tiers I got the words print on an edible transfer sheet and wrapped bands of this around the cakes. Worked very well, with a hell of a lot less stress!
If it's covered in fondant, have you thought about hand-painting it? My handwriting in RI is terribly sloppy. lol If you find a nice typeface you can transfer the message to fondant easily. I find a fine tipped brush to paint makes it easier to cover up boo-boos.
I can only speak from my experience. I have never done a fondant cake so I am not sure how hard it is to write on sides of fondant covered cake. I have written on top and sides of buttercream cake with buttercream icing and it was quite easy to do. I always add a little bit of corn syrup in my buttercream which makes it more pliable and less likely to break.