Well now I've got to make a monogram cake topper for a big wedding cake. It's a 'S'
I'll be using silver peal dust.

I'm planning on doing this with royal icing. I've seen many on the site made with gum paste. Unfortunately I'm not experienced with gum paste at all.

Anyone have any experience with the two... which would be the best?

Any tips, tricks, suggestions would be greatly appreciated.

I would prefer gumpaste, myself. You just roll it out, cut out your monogram and allow to dry. It seems more sturdy than RI to me. But maybe I'm just less afraid of it.

I agree with karateka, royal is way too fragile. If you do use royal I would make several as there will no doubt be breakage.