Animated GIF is better in my opinion. All browsers support them and they're less resource intensive. I regularly visited one site until they added a new Flash banner ad which consumed 75% of my CPU resources (that's 75% of TWO cpu's in a dual-core machine). It was stupid and irritating to see everything else I was running using a total of 5% of resources but one animated banner in the browser hogging another 75% just to get my attention. It worked, it got my attention so I could give the site my farewell kiss-of-death. Flash should only be for things that people seek out on purpose such as videos, games, and so on.

Kevin