The 1.4-litre MINI engine could be pretty troublesome - high oil consumption and timing chain issues are common, which might explain their attractive price tag on the used market. The Fiesta, meanwhile, is known to be fairly dependable, especially if you get one with the 1.25-litre engine. It's difficult to say what ULEZ regulations will be in the future, but I wouldn't expect a huge difference between a Euro 4 and Euro 5 petrol car.