I think it's probably the one that gets the most clicks, but it could be the one with the best paying ads.
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.
higher income depend on content(argument) of your website.
I've been using A/B testing with these two ad formats embedded in the middle of my articles for a while now, and I can tell you that both have the same CTR, eCPM, clicks and earnings. There are minimal variations but in the end both get the same results.
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.
Thank you for your responses.
I believe I have a better picture.
I found there is little difference so I switched to the smaller one. I float the ads to the right and the smaller ad leaves a little more room for article text to the left.
Thank you Ann, that's what I'm doing, switching to smaller 300X250
300X250 has been promoted by the Adsense Team.
I saw a lot of multimedia ads on this size.
I've run tests about this too.
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.
I think the larger box generated more revenue when I tried both of them a year ago. I guess it depends on your site. Try both for a month or 2 weeks if you cant afford a month. There's no right answer.
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.