Hard question when its general like that.
SEO always beats banners for cost-efficiency, but with a big enough budget banners can get more traffic to a site than SEO can, and certainly faster initially.
Some terms are very rarely searched, yet when the same thing practically hits you in the eye at your favourite site, you may be interested - its just human nature.
As a general rule though, SEO produces far more 'targeted' traffic, better quality referrals than the tiny, general pitch of a banner can do.
Industry standard banner click-thru rates are still only around 0.5%, (though particularly good banners can get over 5% quite easily). Industry standard conversion rates (from visitor to customer) are around 1%, so an average banner must be shown 200 times to get a click, and 20,000 times to get a single sale...
(All figures based on Forrester Research statistics for last year)