Both have their own advantages but I would like to say SEO is the better option because:
SEO: Search Engine Optimization: Free of cost, takes time but give good result, continuous and slow process.
SEM: Search Engine Marketing: Costly, fast process, instantly gives result but if you stop paying to search engines, it will cease the marketing of your website, therefore it will lead to dropping down of your website's rank.
So SEO is the Best.