jimbanks - 8:22 pm on Jul 1, 2013 (gmt 0)
As a publisher there are many more advertisers who opt for the 300x250 so you might find there is more competition for the inventory which might lead to higher CPC's which leads to higher revenue.
Is this on the basis for which you might make most money from?
Size is less important as placement on the page, blending of the ad units into the content and other factors.
As an advertiser there are greater opportunities to capitalise on 336x280 ad units, that tends to be the size I look for the most on the Ad Planner.
Don't forget the technical name for the 300 x 250 is MPU, which is multi-purpose unit, it is the size that you can fit text, image, flash, video ads in.