ooh good question! Seo in my opinion is a long term solution and takes a bit of time to get started, on the other hand PPC is a bit more pricey but its instant traffic which could mean instant sales. You should judge this based on what your niche is and what your goal is.
Lets say if you have a seasonal product for example a valentine product then after valentines it would be useless for SEO until next year, but with PPC you can start banking for it now and until valentines day then stop.
Thats just my opinion, I think more experienced members can shed some more light in this matter.