SEO and paid advertising are two different forms of marketing. In a ideal world they should both be run in parallel and at all times however sometimes a clients budget might prevent this. The benefit of SEO (Search Engine Optimisation) is long term but, equally, results can take longer to be generated. In theory, after a successful SEO campaign you can stop and it can still work for you afterwards with no investment or you can ‘stop and start’ it to maintain a position as a good campaign gets you to rank ‘naturally’.
Paid advertising however is when you pay to be somewhere, such as at the top of Google, Bing or Facebook, in this instance it is great as you can instantly drive traffic to your website. The downside of paid advertising is, once you finish, you no longer get the traffic to your website.
As a result it depends on the client as to whether paid advertising or SEO is best. Both of them increase revenue and turnover so should be done for long term success. If you need instant results though paid advertising is great as it can make a significant difference within a very short period of time. If you are looking for a long term, sustainable, strategy though your marketing resources should be spread across both marketing options.