Stavros is 100% correct. They're both Zetecs rebadged as Duratecs. The ST170 has a zetec with a variable timing head, and the RS has a turbo'ed Zetec rebadged as a 'Duratec RS'

Proper Duratecs are great engines, some nicely tuned examples are found in the kitcar world, and yeah the Mazda MPS cars have the 2.3 Duratec with Turbos strapped on. My ST has a proper Duratec engine as well. (please no one try to contest that)

Duratecs have chain driven cams and compared to the Zetec, the exahaust and inlet manifolds have swapped sides (i.e the exhaust is round the back of my engine, compared to a zetec where it would be round the front)