Each methodology has its advantages and disadvantages. SEO is a long term strategy to rank and gain visibility over time. Where PPC or SEM is a quick way to gain visibility online. Your ads only stay up as long as you pay where SEO once achieved and maintained will continue to deliver traffic. PPC can also be cost-prohibitive for small businesses while a well-optimized campaign can bring immediate results to a business since you will be visible immediately where SEO takes time to work.
Some businesses decide on a campaign that is a hybrid of the two. In doing so you will work on your long term goals with SEO and your short term goals with a well-optimized SEM campaign to get immediate visibility and results.