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Now, if you bid on more general keywords such as "half inch widgets" you have no choice but to make them land on the most appropriate category page but your conversions will be lower.
As Netmeg says, if you know specific make, model, size, color or whatever, send them there.
Depends on your ad.
Landing page always echoes ad.
"Biggest range of widgets in the West" --> category landing page
"Best biggest blue widget on the web" --> product landing page
Which type of ad (category or product) is getting the best response?
Which type of landing page is getting the best conversion?