Here is a quick guide to the differences:
Pace ADS200 (120W power)
All metal construction station, stand and handpiece, easy to use interface, clear and easy to read display, cheap and reliable tips which come in two series standard, and ultra (which have more mass). The handpiece is made from one piece of extruded aluminum, it's very robust, cool running and swapping tips is easy. Pace just released new MT200 tweezers for the station.
Hakko FX951 (75W power)
Plastic station, part metal stand, plastic handpiece, poor interface, small display, tips ok reliability but more expensive than Pace. Only takes standard tips. Handpiece is overly complex and feels like it could easily break while swapping tips. They have a dumb tip swapping method which requires you to buy more (dirt absorbing) sponge/plastic sleeves so there is a hidden expense. Has a key card system that will lock you out if you lose it and it sits right below buttons, the station beeps at you as well, both of these features are quite annoying. No tweezers run on the FX951.
So in summary there is no real reason you would buy the Hakko. The Pace is faster, more powerful, cheaper to run and easier to use.