A day ago, I replaced two of adsense ads with pure Text ads. At the time, I read some postings said that Text Ads provide more information/items than Text/Image ones. It looked right to me at the time since Text ads usually provides 4 different items. After I did this, my revenue was drastically lowered even the visitors increase. Then I browsed my site for a few times and found the damn text ads always showed some damn "MapQuest" stuff and almost never changed. Today, I replaced the text ads with Text/Image again, revenue went up as I expected.

I think it may be because advertisers may prefer to have "interesting/distinguish" ads than the dumb text ads. Google also seems to put dumb text ads rather than related ads... anyway that is my findings. If you really want to check if what I said is true or not, you can check elookinto website. I found now it shows much more related ads than the dumb "MapQuest" stuff.

My advice is that you do need to pay attention and do some trials to find the differences.