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This will be totally dependent on your site and your niche - others may have views, but your mileage will vary! If you need to find out, set up an experiment to see what works for you.
Obviously I'm talking about a month's result, some days one format performs better but at the end of the month they are just equal.
For me, 336x280 gets more clicks, and thus higher CTR (~0,5% higher).
But the end income is pretty much the same.
So it seems, that clicks in 300x250 adunit are worth more, which may be results of the fact that there are more ads in that good old format.
However, in terms of position I could say that I usually get more clicks when I place my banners on the left side of a page. Try to blend whichever box you use.