Not sure about all of them, but JB are turned brass with a black nickel finish.
InterMountains are nickel silver with a black nickel finish.
The old original Kadees were zinc... which eventually gets pitted, and picks up crud.
They're all conductive... most Kadees have plastic axles, and need wheel wipers for lighting. Most others have metal axles with one wheel insulated.
BTW, although Kadees have plastic axles, which theoretically could be an issue, I've never experienced a bent or warped problem, and axle points haven't shown to wear appreciably. Other's mileage may vary.