PPC should be used to generate immediate traffic. More time is spent in SEO and it has more permanent effects than PPC.
You have that back to front! How can you start seo if you don't know what keywords convert? Use ppc to work out what keywords convert (and which even have traffic), then use seo to rank for them.
SEO is our approach, although with some clients we have done a combination early on so they would start getting traffic while they waited for the SEO to start taking effect. In addition to the obvious cost difference, users are also more inclined to trust an organic result.