Chocolate used for decoration
Melt the kind you want. Sugar doesn't melt at the temperature chocolate melts, and wouldn't dissolve in chocolate anyway. Commercial chocolate bars are smooth because they are conched. If you mix chocolate with sugar, it will be sandy. 

Alternatively, you can glaze with a ganache, and dissolve the sugar in the cream first. But it is still better to make the ganache with the type of chocolate you want, instead of adjusting sugar or fat.