Since your client did not specify I would do what you think is easier for you. Me, I would choose a tiered cake because I have an easier time covering smaller cakes than a big sheet cake.
Since the way I price I am not sure if the tiered ends up ore expensive or not. I charge for all my ingredients and supplies and add in an hourly rate.
I hope this helps you decide which way to go. Just remember do what you are comfortable with :)
