Hi Jack. They are quite different cars – the MINI will be more fun to drive but the Yaris will be more practical and quite possibly cheaper to insure.
I’ve had a MINI Cooper R50 for about six years now and it’s pretty much run like clockwork. But then again, Toyota is famed for reliability and the Yaris is no different.
Have a drive of both and see which suits you better. And make sure you have a read of our Ten Golden Rules for Buying a Car.