Consider the other costs. Under no circumstances can any roofing material be installed over the top of existing cedar shakes or shingles. Cedar roofs are more prone to leakage than asphalt. Cedar rots, asphalt doesn't as quickly. I would walk on an asphalt roof with much more confidence than a cedar roof, which is 10x more slippery. Your homeowner's insurance will be different with the two different types, check with your agent.