If your potential client is after ROI then sponsored listings offer a good way to determine how much your investment is costing. SEO is more difficult to determine the exact ROI, but it can be done. (Time needs to be factored into both SEO and Sponsored listings) SEO may prove to be more cost effective for high traffic keywords.
If your client wants Brand recognition then you could split sponsored listings into smaller categories: (1)Pay-per-click is great for branding purposes, unlimited impression at 0 cost; (2)Adwords and other programs that use CTR or Pay-per-impression are good for branding but not quite as cost effective. SEO is harder to use for branding, depending on the SE you may not get a great description for your site. You would then have to focus on brand impress when visitor actually hit your client's site.
You need to know what your client is after, in order to help them know what combination is best for them. Time may play a big part of their business plan. How soon do they need to see results.
Personally I think that SEO, Pay-Per-Click, and Pay-per-Impression all work together. Each targets a different market segment. Also remember your client's marketing objectives may change over time.
Websites are created with different purposes in mind, but bottom line is if they are paying someone to do help with their online marketing they are probably in it for the bottom line: overall Profit.