I would recommend you use both. PPC is faster but it does take some research and usually website mods to avoid wasting money. It can also get very expensive. SEO does take longer to see results, at least 60 days and depending on the competitiveness of the terms you target it could take a lot longer.
The advantage of SEO is once you do it right the results are long lasting and will not cost you a fee per click. There is a good article explaining in detail pay per click and ppc called SEO vs PPC
I would do SEO but use PPC doing the time it takes for the SEO to show results. As my keywords start showing up in the natural listings I would than delete those keywords from the PPC campaign.