Yea, in terms of valuation, PPC would be better than
SEO because it is a venue you have some modicum of control. Your costs are relatively known and you can project the average annual increase of these costs within a decent of margin of error. You can't project
SEO and the impact of any future algorithm changes. Of course, I'm not saying ignore
SEO, but from a valuation standpoint this is how I would evaluate any company and their traffic metrics. Buying a company with $X revenues and all traffic via
SEO would be, IMO, high risk. Incidentally, when I was in selling mode with my company the knowledge of my "super-affiliate" would cause suitors to back out of the deal -- this was because the variable was uncontrolled and in effect, subjected me to a Commandment of Control issue in a backwards way.
A similar example: If you manufacture a great product (YOU HAVE CONTROL) but one of your customers is 30% of revenues (WALMART) you are subject to a Commandment of Control issue in reverse - you're controlling manufacture but are losing control in distribution. If Walmart drops ya, you're in trouble.