Assuming your talking about Adwords, Optimizing your site for search engines would be better. Organic traffic is free. Adwords (PPC advertising) cost money.
he wants to know where to invest: adwords or organic seo? that is a really good question. i would lve to hear some answers also
Invest in the SEO. Once you are ranked you are there and you never pay a cent if someone clicks on your link, even if you get 1,000,000 organic clicks (which would probably cost you a million dollars or more with adsense). Just know the right places to go. I've used one service that was $100 for a guaranteed page 1 google rank and he delivered (msg me if you want I can get you his info). You could easily spend $100 a day in adsense and get nothing back!
Assuming you are talking about Adwords, For short term and initial stages of your website, Adwords is good. But in long run, organic search will be helpful to you in large proportion as edge83 has said.