Driven all the STs. Own an RS and a FiST. This is the best I can describe your choices:
Focus ST (FoST): A front wheel drive mustang. Heavy through corners, fast acceleration, slightly more usable space than the FiST, smoother ride than the FiST. Better for drag races at a street light.
Fiesta ST (FiST): A front wheel drive lotus elise hatchback. Light, nimble, great handling, super responsive steering. Slow acceleration (feels fastish - but isn't), bumpy ride quality. Horrible for racing at street lights. Must be willing to peg the throttle.