Additionally, there are many, many things you can do for SEO which don't require your product to exist. You don't have to have a solution available to show to be able to, e.g., blog about the problem domain, create resources which solve problems for the people you hope will eventually give you money, start pushing the emotional buttons of folks in the niche, etc.
AdWords, on the other hand, you can put off until you have reason to suspect that you'll be able to do it profitably. (If you can't make money with a customer acquisition cost of zero from organic search, it is unlikely you will be able to make money on AdWords.)
I pay my outsourced employees who do SEO for me by the hour, so maybe I feel the financial pinch of a large scale SEO campaign more than others, but any time spent on SEO by anyone in your startup is money.
In the past several years I've staked significant money(6 figures) on my ability to predict traffic quality(how well a traffic source converts) and you may find that the people who click on ads(sometimes before clicking on any search results) convert much better than SEO traffic. I've seen 50-150% difference in conversion rate between targeted free and paid traffic several times.
This is why many companies bid on their name in their SEM campaigns, even though they obviously easily rank first for it organically.
This isn't necessarily always true, but it's something to think about.
Do you have any marinating recipe tips?