I’m genuinly surprised nobody compared the two before!
There are many similarities in their sense of humor: both use wacky, insufferable characters in a world that is much more like ours than the first glance would reveal. And they use those characters to tell us something about our own world. The big difference is: Pratchett loves his characters deeply, and none of them is a cartoon character: they all have real personalities. Heck, for every sidekick ever introduced in any of the Discworld books, there’s another book where this character plays the lead
Zaphod, on the other hand, is so one dimensional, I’m sure it took Adams a lot of WORK to avoid any character development! It feels more like a literary experiment than a story. And call me crazy, but I prefer a story :
PS: Truckers is to Discworld what “the hobbit” is to “the lord of the rings” !