When I was buying my car I went for the ST opposed to the cooper s, this was mainly because when I was buying my £12k got me a brand new ST but £12k would've got me a 52/03 plate mini with not a lot of kit and lots of mileage.
Things have probably changed now though with more cooper s's on the market for less.
Do I regret getting one instead of a cooper s? No way, it's still an awful lot of fun and enough power for me (at 18 anyway) and I have since had it lowered with some eibach pro springs and handles like a dream. Although spending more money people with other ST's have managed approx 190bhp with a new exhaust, manifold and decent induction kit (probably for around £800-900) but then again you might be able to get similar gains with the Mini with the same spare cash.
Build quality wise I wouldn't say there's much in it and the fiesta has lots more room than the Mini, but the best thing to do is just test drive them both and see which you prefer
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