Right, just had a phone call back and basically both engines are 5 cylinder in-line 20v turdo, but the S-Max is tuned to 220bhp,and the ST isto 225bhp.
The other differences apart from the engine tuning is the gearbox, as although both have a 6 speed the gear ratios could be different too. So it is likely to be a completeswap of the engine, gearbox, and plenty of wiring from the loom I expect....
So Richard, you're probably right in saying that the best motor to go for would be the one from the S-Max.