I own a couple of VWs - a Corrado VR6 and Beetle. Have owned an 85 GTI as well. Friends are S4, A4, Jetta 1.8T and Corrado VR6/G60 guys so I get to ride in the whole spectrum of the VW/Audi group in the US.
Anyway, the build / interior quality of a new VW/Audi cannot be beaten. No contest there. But the MINI comes close. Where the Golf blends into the landscape and may look a bit bland, the MINI is interesting. So, in my mind, it is a small trade-off in quality (very small) for a more interesting / fun car.
Fact of the matter is that (in the US at least), VW has gone decidely away from the sporty nature it used to have. Also, I think the interior of the 'S' seemed a bit higher quality to the touch / eye given all the metal bits in place of plastic ones in the Cooper / ONE.
Jason