Instant traffic via PPC engines is great, but test ferociously and track your results ferociously. And run your tests of the different PPC engines as scientifically as possible–use identical ad text, keywords, positioning, etc. Then calculate your conversions and ROI from each engine.
It’s work, but it’s worth it.
Traffic from search engines is great if you can get it. There are tons of forums on the web regarding SEO techniques.