RX100 dual pivots are great brakes. I've been riding one of these on the front for the last few weeks. Mid 1990's vintage; they appear to use the same geometry as the great Dura-Ace 7403 dual pivots.
The finish is good, the hardware is good, and they are stiff and powerful stoppers. The stock Shimano brake pads were horrid, and seem only useful for covering your rims in black muck during wet rides.
RSX is a different brake.. Different castings and much cheaper hardware and finish than RX100. Nevertheless, the RSX brakes are powerful stoppers as well.
I've used all of these, and the nice Shimano 600 dual pivots, and the contemporary 105 brakes. They all perform about the same, with the same setup and pads. They are all far superior to the single pivot brakes that preceded them.