It depends on the business, but generally, it should be both, not a choice.

People get all giddy about "inbound marketing" and think SEO is the holy grail. The problem is, that in competitive markets, everyone has that same brilliant idea. SEO agencies tend to promise more than they can deliver. The reality is it's impossible to guarantee a top ranking for a non-branded query. It also tends to take longer than people think to achieve the top spot.

And PPC, well, it can be expensive. Without free cash to invest, scaling PPC is tough, especially in a business with a mid-long sales cycle.

Now that I've shredded both, here's my view:

You need a strategy for both. When you build a SEO strategy, you should be thinking about NEXT year's sales and the long-term growth of your business. PPC and targeted social ads are the more effective way to scale near-term sales. In the ideal world, you can slowly move away from large paid investments as organic scales.