The DVD just has a higher quality of the ISO.

is the dvd bootable?

How is the quality of the content of the iso be different on cd or on dvd??

It was a general fact with out getting long winded. Yet, here is the difference.

www.imation.com/products/dvd_media/differences.htm

The DVD includes both Ubuntu, Kubuntu and Xubuntu. If you don't want all three
there's no reason to ask for a DVD, a CD will do just fine.

I didnt find the link provided by gobby but found this one.
www.imation.com/.../differences.html

DVD is the better media but it depends on what CD drive you want to run
it in.