Yahoo! Auto Canada compares the two and favors the Ford:
Put a completely different—and blunt—way, the MINI Cooper S feels like a car that will be more likely to impress your passengers, while the Fiesta ST will impress you specifically.
If you value more creature comforts, or want more customization, you might want to hold out for the MINI Cooper 4 door. Otherwise, the bottom line is that if your budget is limited, without a doubt go for the Fiesta ST. It shows what a hot hatch should be today.