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And generally, the distribution of PPC verus PFI are different, so you are extendign your reach.
There have been some studies (can't find the links right now) which say that some people click on ads, whilst others click on search results. A recent study also found great benefit in being in both results.
So, I would say, do a balance of whatever your budget can afford. Track the results (PFI can be tracked), optimize the campaigns and see what happens.
Compare that with £1500 PPC spend on Adwords - is there really going to be that much difference in clickthrough.
I don't think there would be. With PFI, there is no account to be charged every month, so there's a clear advantage to be had.
But, if you can optimize for a #1 spot with PFI, it is likely to have a much greater ROI than keeping that #1 via PPC. But, it also makes that ROI dependent upon an algo.