There has been an ongoing debate among marketers on which is better of the two. Many in-house marketers support SEO, and rightly so. However, it’s worth noting that true SEM is ineffective in the absence of natural SEO.
On the other hand, there are cases where PPC, a constituent of SEM, is more appropriate and effective than SEO. A good example is when launching a new site and visibility is needed immediately, a PPC campaign is the most viable option since it yields results faster than SEO.
Even then, it would not be prudent to use PPC alone without giving SEO a chance.
SEO may be slow to give tangible results, but ultimately, it proves cheaper and builds a better solid search credibility compared to PPC.
The choice of whichever tactic to utilize should be based on the consideration of the particular needs to be met, but even then, you should understand the discrepancies and how you will sustain your efforts.