My opinion is the SI is going to sound better because its DOHC and the mini cooper is sohc supercharged. And the only real way you can sport out your mini is with John Cooper Works stuff which is 5K for 37 more hp and 22 more lb/ft torque - via bigger Eaton supercharger/exhaust/ high flow cylinder heads - but do you like supercharger whine? And the SI weighs 200 lbs more. SI's are more upgradeable but coopers are trendy. Just stuff to think about.