This is quite a question and one that I believe depends on what market you are in and what you need to achieve from your website, if you have a long term view and you are time rich but cash poor then Search Engine Optimisation (SEO) is with out doubt the way to go, you will not see dramatic results over night but in a few months you can get significant extra traffic by ensuring your site is set up properly for the search engines to crawl through and index.
If however you have a good budget and need to see immediate results then Pay Per Click (PPC) may be the best way to go, it will send visitors to your site from day one and with the statistics packages available you will be able to see the ads that resulted in conversions and the ones that were ignored by visitors.
This brings us to another important down side of PPC and that is that Ad blindness is worse than ever Comscore, the marketing research company shows that 16% of Internet users are responsible for 85% of all ad clicks on the web, so the vast majority of visitors from your PPC ads may well have clicked before, this may not be a huge problem if the sales levels are good but it is a disturbing trend.