They are both good, it depends on the person that wants to use it. Personally though I prefer SEO because the traffic you gain at the end of it remains, it might just take a little while before you start getting the result. But SEM traffick result will disappear when you stop using it.I think SEO is better because you expand resources once and reap benefits forever because once you achieve success in your SEO, it is a formidable competitive advantage that is not easy to replicate in a short period. And it is expected that in the long run, the cost per visitor will be a small fraction of what you will end up paying for SEM campaigns.