Advertising buys traffic. SEO builds it. The real difference is what remains when spending stops.
The day you cut the ad budget, traffic goes to zero that day. The day you stop SEO work, traffic continues for months and then slowly declines. One is an operating cost; the other is an asset.
That does not make SEO better. If you have a product and do not yet know who buys it, advertising is the fastest way to find out. A few weeks of ads can disprove a hypothesis you were about to spend six months of SEO on.
The sensible order is usually: use advertising to learn which message and which audience work, then use SEO to turn that into a long-term asset.
One warning: SEO on a site that is slow, structurally messy, or broken on mobile is money thrown away. Technical SEO comes before content production, not after it.