I tested Z Comet and Gl Fuse (both max weight) a few weeks ago and tried to figure out the differences.
I didn't notice that much difference between the two, but here are those that I found…
Fuse was a little faster and worked better for driving. It was actually faster in every aspect, it did what it did faster. When it turned it turned quicker. When it faded on slight hyzer throws, it went down faster.
Comet made more extended turns and faded more slowly at hyzers. Very good for long curves, when you need to stretch out the curve. Perhaps it's the larger diameter that makes long gliding curves easy. Comet also worked a bit better for touchy shots.
My conclusion was that Comet was the more unique of the two and found a place in my bag. But it hurts to put the Fuse aside, because I want it so badly! Maybe a Fuse in Opto would do the jump into the bag, but I have only tested it a few times, so for now… Z Comet is in, Fuse is out.