Hi Christian
You probably need to define your advertisement a bit more, you say its a banner, so I have to assume it’s a std 280 x 60 type horizontal format. But you may be thinking of a 400 x 90 type . please give me a bit more to work with, including where YOU think it should go. Have you thought of an image advert in a module position right or left ( vertical image) rather than a banner? show me the banner, show me the site, so I can see the colours, texts and get a feel for the balance or unbalance it would bring to the site?
For a professional look, I feel balance is important some disagree 🙂
Blessings Shannon