Purely from a driving satisfaction point of view, which is more pleasurable to drive? The GTI has a higher quality interior and the dashboard is much nicer.
Is the nice throaty growl and the Recaro seats of the Focus ST enough to make it a more fun drive?
To echo Bentin, stock for stock, I also think the ST is more fun to drive.
For me it came down to this:
GTI = More refined driving experience. Quieter, more comfortable, nicer interior... But all that smooths over the engagement/fun factor. It's a more laid back car.
ST = Lots of engagement and fun; definitely has personality to it. But, it has a worse interior, comfort, mpg, etc...
If you're thinking about modding, which car's weaknesses are easier to "fix"?
Well, It's alot easier to make the GTI more fun and engaging than it is to try and improve the interior, add sound deading, and improve the mpg of the ST. Some of which you can't even realistically do.
Hence why I'm in a GTI now. If you're keeping it stock though, I can see why someone might choose the ST instead.