WilliamBlairpinc.com

Under identical circumstances, a Flash banner ad will perform similar to a GIF banner ad of equal quality. That being said, Flash banner ads overall perform much better than GIF animated banner ads because there is much more you can do with them. You can add interactivity, audio, video and much more to a Flash banner ad. You can also increase the amount of text and imagery while staying within file size limits due to Flashes ability to compress better than a GIF ad.