If you're building a serious business, you must do both although if your adspend budget is right, you can ignore SEO and buy your way to the top.
If you're building a sham affiliate business or making a content-farm splog, SEO is your only hope since SMO involves interacting with human eyeballs that are awfully good at recognizing garbage when they see it.
smo is not as easy as seo as it is a long term strategy. you have to grow your network in your niche and build a good reputation. only then can you influence results gradually over time
SEO is difficult, but good for targeted traffic. It helps in achieving search engine rankings, which SMO can't. SMO is easy to perform and provides high traffic towards website, but you can't expect much quality(targeted) traffic through SMO.
These days, I would say both are pretty important and contributes in increasing the traffic on your website. If you practiced any one of these strategies, then you will not get maximum results. One has to employ both of these tactics simultaneously by linking each other.
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