AI entered into a cake decorating competition and I want to know should I use royal icing for decorating or a crusting butter cream.

It depends on the competition...if there are rules regarding what medium to use, then you have no choice.

If you do have a choice, consider...will the cake be on display for more than a day? Even a crusting buttercream will start to degrade after a couple of days. Will it be hot? One year an entrant at my state fair had her buttercream melt right off her cake as she walked it to the competition area from her car. It was a sad sight to see .

AHi, Everything is done on cake dummies. Not rules on mediums. They will be on display for 2-3 days. I was thinking if using royal icing

Covering a dummy in RI is .......................a PITA.  I avoid it at all costs. 

I'd go for fondant with royal icing decoration.