As the topic says, what's the difference? Which one should I use?
DiscSpeed is the most recent (and updated) version of CD-DVD Speed.
For what I know, there are no big differences between the two versions, only updates and bug-fixes
Maybe some little changes in the user interface, but nothing to be worried about
as said on the website: “CD-DVD Speed” has been renamed into DiscSpeed.
According to these threads, DiscSpeed is buggier than CD-DVD Speed and also lost support for jitter testing: