i have 2 st170s. a road and track car. so the choice is obvious from me. get the fiesta lol.
although i like my cars they have loads of common faults, are heavy on fuel and not that fast. if your handy at fixing little niggly things and must have a st170 then yes buy one. if you prefer the fiesta id say go for that. iv never owned a fiesta st though so no idea what they like.