Which one is can get you more ROI ( Return of investment ) to a retail buiness website? Search Engine Optimization or Search Engine Marketing ( Provided Business website is in top10/first page of SERP)?
If website is already on 1st page in SERP,obviously SEO is going to help none or little. Targeted SEM is the right way to bring more customers.
SEO is part of SEM. SEO should be part of long term plan. Other then that ppc could help getting instant sales.
With Search Engine Marketing, you can increase your ROI, but without the help of SEO, SEM will not be successful.
Webcosmo is right. SEM is a bigger topic. It involves both paid / non-paid traffic. SEO is mainly about non-paid traffic only. Ideally you should work on your SEM so you get the best of both worlds