Have never played a JSX, But like everyone else has already said its basically just a XXX modded. As far as the noise gate on the JSX, the Triple XXX is pretty quiet already. I own a XXX along with a 6505 and the two really complement each other in a mix. The differences are the resonance and presence controls along with fat switches for the gain channels. The crunch channel on the XXX is basically the same thing as the ultra channel on the JSX (can achieve the same amount of gain). My vote is stay with the XXX. I believe Everybody's X already stated something similar: its like trading a 2000 Ford Explorer for a 2000 Mercury Mountaineer (different knobs and switches but the same frame).
p.s. put a BOSS GE-7 in front of the 6505 crunch channel and you can achieve "usable" clean tones.