I looked at both and test drove both. Went with the Fiesta ST.
Was a bit torn about the interior in the 208. Whilst the cabin felt slightly more "upmarket" compared to the Fiesta, the seating position/steering wheel position was not as good. Also the seats in the Fiesta were loads more comfortable and better looking (they look like an afterthought in the 208).
The Fiesta also had some nice touches like voice control for music and keyless entry/start that the 208 didn't offer. The prestige does have some things the ST doesn't have as well but that's only cornering fogs and a panoramic roof I seem to remember. It does have touch screen nav and the ST's nav is not touch screen but then the 208 just follows the horrible design of just sticking a tablet screen onto the centre console which I think looks naff. Neither of which I wanted. Keyeless entry and tart however is a revelation - probably one of the best options/gadgets on a card I have had to date. So nice not to have to faff about with keys.
and then to the driving experience...feels more of an event in the Fiesta. The whole car pivots around you unlike in the 208 and whilst the suspension is harder in the Fiesta, it certainly pays dividends in corners. Also the sound symposer in the Fiesta is a great little extra and sounds great. Nice to hear it growl at you all the time
They are both good but the Fiesta is a better hot hatch in my opinion.
The 208 gti has just been updated and I haven't driven that one, although I think the only difference is slight facelift and a different engine putting out 208bhp and 300nm of torque.
Oh also, don't forget you can get the ST mountuned for £600 so it has 215bhp, 320nm of torque and you can still keep your warranty intact as it is Ford approved (it is even in their brochure for the Fiesta)