Text ads for me, but after I read the post above maybe I should try a text and an image ad side by side.
It depends on what type of website you're running. For example, I run BOTH on webmaster related websites.. I run image ads only (and some link units) on entertainment based websites.. You really have to think about your target audience.. It's impossible to say which are best in general.. It differs for different niches. Chuckun
I started a discussion some time ago with some insights here: http://forums.digitalpoint.com/showthread.php?t=1927148
The way to look at this is what gives you more options to customize your ads, in text you can customize it to match your website which will make it easy for people to click.
I can only go on results for my sites and it is definitely text. Try experimenting with them yourself.