The answer here changes dramatically based on your target customer.

If your company is b2b, then SEO and Adwords is most likely not going to achieve the best results. Most businesses are not actively searching for products or services at a high rate. You'd be much better off spending your resources on outbound email marketing, PR, and increasing the conversion rate of your traffic and customers.

If you are b2c, in a market where consumers are searching for you product/service then SEO is most likely your best long term strategy. The reason is because you can think of SEO as investing into your website as an asset. When you spend money on advertising, the money is gone after the ad runs. But with SEO, you are building into a website that will stay valuable for the long haul, especially if it is producing leads/sales.

You would then be wise to make the most of your traffic and it's increased rankings through a sales funnel. Make sure you give them a hook once they hit your site. The action you want them to take should be very clear, even in a fraction of a second.

For visitors who leave your site without taking the desired action, you can bring them back using a combination of retargeting and email marketing for low cost customer acquisition.

Put it all together and you'll have a customer acquisition machine that runs on autopilot! Now you can focus on the other 1000 tasks involved with scaling a business :)

Reach out to me to hear how to specifically implement customer growing techniques. Would be happy to help.