Pay-Per-Click
This isn't really a question of Search Engine Optimisation versus Pay-Per-Click. Nor is it a matter of Natural Listings versus Sponsored Listings. The important issue is how to combine both fields in order to get the most traffic, relevant to your website, in the most sustainable and cost effective manner.
It is true that Sponsored Listings can convey an image of professionalism and this is something SEO can't replace. However, Sponsored Listings have many limitations that can only be overcome by top Natural Listings.
Availability - Natural Listings are always on. If you set a maximum allowance in your PPC account and this limit is reached on the morning of a very busy day, your ad will not be displayed for the rest of the day and you may loose some promising leads. A natural listing on the other hand, does not require any form of payment - it's always displayed no matter how many clicks/visitors you get.
Exposure - Natural Listings are everywhere. With PPC, you may have to set up several accounts with different companies/search engines in order to advertise with them. However, this sort of restriction doesn't apply to SEO. A well optimized website will rank highly on any and all search engines.
Cost - PPC advertising works on the basis that you pay for every visitor you get. The more visitors you get, the more you'll have to pay. In contrast, SEO is generally conducted as a one-off project and requires little maintenance. So once the Optimisation is complete, you will continuously get more and more visitors at no extra cost. This is why SEO is in most cases a more cost effective way to increase the number of visitors to your website.