The big difference: Fleet is a standalone novel set in Known Space. If you have a vague memory of Ringworld and the short stories, you’ll be fine. Juggler, on the other hand, is not only a direct sequel to Fleet, but also to more than a dozen Known Space short stories (including everything in Crashlander), which are air-dropped into the story at seemingly random intervals. The “big surprise” is also so poorly handled that for several chapters you can’t be sure that a major character wasn’t just another reference. On the plus side, it gives a faint nod to the old “down in flames” story idea, which was more fun than most of the backstage views of decades-old stories.
I’ve read all of the Known Space stories several times, so I got most of the references, but the secret history of the secret history of Beowulf Shaeffer just got tiresome after a while, and it took up space that would have been better spent expanding on the interesting new characters who were the focus of Fleet. They’re pretty much reduced to spear-carriers in Juggler.
Fleet stands alone quite nicely, and adds some real depth to the Puppeteers, both individually and as a race. I didn’t dislike Juggler, but I doubt I’ll reread it.