The JBC has adjustable temperature, which lets you solder faster if you crank it up. The risk is that you will cook either the board or the component you are attaching to the board. This decreases the reliability of the finished assembly, and it is not always visually obvious when you have cooked something. So it is like the gas pedal on a Ferrari, dangerous if you overdo it.

The Metcal does not have adjustable temperature, the temperature is set by the tips you choose. So it only has one "speed" and this reduces the risk of cooking the board.

Both Metcal and JBC are good, ultimately it turns into a sort of Vim vs. Emacs sort of thing, aka personal preference.