The Clio would be my choice hands down (not because I'm bias in anyway towards Renaultsport...!)
Much better handling (providing it's a Cup/FF), quicker, probably cheaper to buy although I'm not clued up on the price of used Fiesta STs, and faster. Interior isn't as nice I'd expect though.
My mate bought an ST over a CS because of the interior. I went inside and I really don't know why it was just as bad as the Clio. Which really isn't that bad, unless your really anal. Yes the trim comes off randomly, the glovebox has a gap in it, you get the odd rattle but that really is me being anal.
I live with it bar putting the trim back on correctly and will get round to the gap and re-painting the dash strips. But of course its a 6 year old car that has been ragged to death.