Why has it put you off the ST? It's a completely different engine and therefore a very different drive. Personally I would liek both depending on how I felt (and how far I was driving that day!).
St is different to Zetec S lol... Seats... Engine... Proformance and Parts... Wheels etc. Thats exactly the same as saying just because the Mondeo Zetec S looks like the ST220 u wouldn't buy it or the TDCI Sport Focus Against the St170.
Up to u m8 but it will be a REALLY REALLY good car.
The interior of the ST is much better than the Zetec-S and the exterior is also sufficiently different - bigger alloys, lower and covered in ST badges!
I am sure that the ST is going to be a hoot to drive. Can't wait.