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Google Adwords, for myself, was by far superior ROI for the first year after it was unveiled... but as more advertisers have joined the ROI has dropped.
Another nice feature of Adwords is that good ads with compelling text can result in a lower CPC, thus increasing ROI.
I'd strongly suggest signing up for both Overture & Adwords and try for yourself. Also, give the Google Adwords Forum [webmasterworld.com] a look for more information - there's good reading there that should help increase your all important ROI.