Banner ads do help to grab attention, but if one clicks on the banner ads just for attraction, the conversion may not justify the clicks received for the ad. Hence text ads give more targeted traffic and higher conversion rate.
To advertise my website to get more traffic I would always want to use banner ads. Banners will get the attention of people and they will hopefully click on them because they like what they see and come to your website.
If I am going to be advertising my website then I want to use banner ads if possible because they can be made to draw more attention and catch the eyes of more people.
I would only use text ads if there was not a banner ad option available.