Tough question,
It’s hard.. nearly impossible to answer such an answer without seeing and comparing the actual banners vs. the actual text links.
and even when seen, still hard to answer.
{ 3 lines and yet i didn’t answer your question:popcorn: }
Banners bigger than 468*60 ( I personally like 250×250 ) IMHO are much more eye catching then text as they are colorful, dynamic and most of the time are… Simply Impressing..letting the surfer feels the advertiser/ casino is someone serious.. not cheap.. (excuse my english)
on the other hand.. text makes surfer feel a bit more cozy.. more personalized.. but that really depends on the site layout and other factors such as how many banners are on the page.. is it in an “objective” site or a site with a more personal approach etc.
I suggest to use them both.. some text above the banner and some text below. this gives the effect of both impression and personalization.
but… that’s only my opinion.
Hope that helps