I currently have the 336 box on one of my sites, but I'm not very satisfied with the results, if just because it only gives me the two ads, when there should be 4 ads. 250 will give me 3. What does everyone think? What's your experience with the two formats?
I don't run the rectangular boxes often enough to know if one outperforms the others. According to Google, they will show fewer ads to increase the potential relevancy and revenue to the publisher even though more could fit into a given space. I run a lot of 160*600 scrapers and sometimes they only show 3 or 4, versus the full five. I did place 338*250 box on http://www.freshdames.com/top-searched-women.html and I'm only getting two adds also.
The 336x280 is easily my top performer. Followed closely by the 160x600 and the 728x90 leaderboard. But this mainly depends on your site. This is why we test things. Try the smaller box for a week and see what happens. If it doesn't work out, put it back.
Solid points error10 - also a very good use of the channels I'd say, for testing what works with your site and content and what doesn't.
I'm now seeing 336x280 boxes with 4 ads again, though not all the time -- maybe half the time. But I suspect that the 2-ad ones are paying a lot more than the 4-ad ones. Haven't proved it yet, but it's looking that way.
I did a little test a couple of months back between 336-wide and 250-wide blocks in a table on the right side of my content, and found the 336 blocks to get better results, presumably because they are more visible. Your mileage may vary.