Good question! The download button means that you can download the full casino version to your PC. If you have enough memory, this is a good thing because you can access all the casino games. The Instant Play button is for those players who do not want to download the entire casino version, thereby using up precious memory space. The Instant Play feature allows you to play in a flash simply by registering as a guest. Although it should be noted that you may not have access to all of the casino games using Instant Play.
Casinos offer Instant Play feature: