FST is the superior tech in my opinion. RF is lighter but FST is a lot stronger and has a smoother flex and nicer dampening (in my opinion). I find RF feels more like a standard lightly glassed EPS/Epoxy board whereas FST has it's own feel and flex pattern.
There's others on here who much prefer the feel of RF, but for me if the board is available in both constructions, I'm picking FST. If you like your boards super light with a very snappy response then you'll love RF, but be prepared to sacrifice a fair bit of durability on the rails and bottom.
Pretty much agree with Phill here...FST smoothly distributes that springy feel throughout the board. RF feels like it has more pop, due to its lightness. FST will be more durable than RF which is important to note (depending on how long you intend to keep your boards). Personally, I like RF technology on my small wave boards and FST technology on my better wave boards.
I don't think they are really much lighter at all. I think difference could be in the weighting of the board in different areas. The FST carries more weight in the rails, so the swing weight is further out and wider, where as the RF with the carbon rods the weight is more centralised.
Just my opinion. I haven't seen anything scientific to back this up.
Firewire doesn't really specify RF versus FST weight because boards are hand sanded and laminated so they are approved within a range. Therefore comparing two boards here would be different than comparing 2 boards there. Generally speaking though, RF is a lighter tech but only by a relatively small margin.