If it helps, I've got a ST170 at the moment and I'm thinking of changing to a Fiesta ST.
Like all foci, the ST170 can handle and is fun to chuck around, but and it is a big BUT, the engine and gearbox combination makes sure it isn't a great car to drive. The gears ratios are too far apart, 68mph-ish in second, 180mph in sixth! The engine doesn't feel like 170ps and it really doesn't encourage you to rev it, a bad thing for an engine which doesn't come alive until 4800rpm. Finally, the engine is thirsty, really thirsty.
I still like the look of the present ST170, and the handling is great, but it a frustrating car to drive. I had a 1.8 Focus Zetec before the ST170, it was a more rewarding car to drive than the ST170.
The Fiesta ST like the ST170, on paper, promises so much, hopefully it won't disappoint
Hope this waffle helps.
James