Is this true?

[nickys]

An adsense guy on chat told me
"In almost all cases, the 336x280 will perform better
Our system will look at all 300x250 and 336x280 ads when it runs the auctions, so if the 300x250 will perform better, we'll show that slightly smaller ad in the larger ad unit.
That all-but-guarantees that your earnings on the 336x280 will be higher than the 300x250 alone."

[netmeg]

Well maybe; I've never run a 336x280 myself (I wouldn't have room for it unless I put it right in the content)

Presumably you'd have more image ads competing because that size ad unit would draw from both 336x280 and 300x250, but I'm not sure how it would/could affect the text ads.

[ken_b]

In almost all cases, the 336x280 will perform better
Our system will look at all 300x250 and 336x280 ads when it runs the auctions, so if the 300x250 will perform better, we'll show that slightly smaller ad in the larger ad unit.

I set up every other page of a 1,500 page section to test this when AdSense suggested it to me a while back.

It made no real difference in income as far as I can tell.

I don't know if you can dig down in the stats to see specifically how many 300x250 show up in a 336x280 spot. That might be interesting.

I do notice that the 300x250 float at the top center of the 336x280 box. (Just took a look to be sure I was right about that.)

So if your ad slot is left aligned in a column a 300x250 in a 336x280 slot will be offset a bit to the right.

.

[littlecubpanda]

336x280 is the way to go no matter what.

[SEOPTI]

With the custom ad fromats custom performs better than both of them.

[netmeg]

The 300x250 works well for me. I'd have to significantly change my layout to work anything else in, and they earn well.