Generally speaking, DVD images allow you to select from several GIUs, such as Gnome, KDE, and such. There are also more packages on the DVD images, which allow you to configure the installation for multiple purposes, example: development, web server, and such. Also, it's much better to have a DVD image if you will be using the installation offline. Note that in many cases, the DVD image does not support the live boot environment, whereas the CD image does.