Every product has its own advantages and disadvantages. Enabling Auto mode or both text and banner advertisements would yield better results as it is more likely to earn you better revenue rather than choosing only one of either two. However, the design and content of the webpage also play a role in deciding which format to go with. For example, a 300 x 250 image ad of Google Adsense cannot be placed beside an image block of similar sizes and is treated as a violation. As such, for such cases, text ads will be more useful and safe.
Moreover, there are also In-text advertisements which also works great for most blogs and websites. Therefore, while choosing either banner advertising and text ads, try to enable both if possible or go for any one of them that blends well with your content and also be careful to follow the terms and conditions of the advertising program.
Further, the source of traffic also plays a significant role. Organic traffic from search engines performs well in case of conversion and if any valid traffic arrives from the US, Canada etc., the CPC or CPM will be much higher.
