If you mean which engine is better then it really does depend. The I4 engine in the RS2k is an ok unit for an out the box bit of grunt. IMHO though it's heavy and does nothing for the balance of the car. I would stick with a 2 litre Zetec for turboing as is there is far more scope for power and more people that have done it so information and assistance is more readily available.

What sort of budget are you looking at as a Zetec turbo isn't cheap and then you'll need to allow for suitable management to run it, and brakes and suspension to reign it all in. By time you've done it all you do need to step back and think if you'd do better just changing your car for something else?