depends on the site why not try and see what works i think you can change them around. I like text adds myself when i'm on a site image kinda seems annoying i look at it but hardly ever click.
I am using both text ad and banner ads coz I am getting good results with both ...But you can just try using on at a time then try to assess if what ads performing well in your blogs/site
I use both of them. In content area, i will use text ads and in the sidebar, I've used text and image ads