A fitness trampoline with knee injuries doesn't sound like the best idea to me. Although the trampoline springs cushion and absorb force your knee can still take pressure which can be dangerous. You don't want to redevelop previously existing injuries.
However the answer to your question - A fitness trampoline can be good for your upper body as well as lower, not to mention cardio. Depending on the intensity in which you use it, it does have many benefits.
However cardio is cardio, whatever you choose to do. As long as you up the intensity and push yourself like anything in fitness, results will follow if you follow the basic disciplines, diet, consistency etc.. so whatever you prefer is what you should pick as it helps you to remain motivated. If you want to jump - then buy a trampoline, if you want to run - Maybe a treadmill or even go for a run outside, if you enjoy the cross trainer - then buy a cross trainer etc etc...