Just like you said.
The ST170 still is a simple Zetec-E engine that was based on the CVH which means it's a long stroke engine.
There's four reasons why the ST170 has ~40 bhp more than a standard blacktop:
1. Cosworth head design with bigger ports and valves
2. 4-branch exhaust manifold
3. Variable inlet
4. Variable inlet camshaft timing
The manifold is too big for your Escort. The variable inlet is a pain in the arse to set up, and still most of the aftermarket ECUs won't support such items. You don't really need the VCT. When using the engine in your MK3 Escort you'll give a f*** about rough idle and less low rev power with a pair of sports/fast road/race cams. So the only really benefit is the cylinder head. It's much better than the heads of the silvertop/blacktop engines but this doesn't kill the fact that it's still an engine with a VERY conservative design.
The ST170 is easier to fit but the Duratec has a LOT more potential than the Zetec-based ST170. Weight yes, but that's just a minor point. The whole engine design of the Duratec (including head, ports, valves etc.) is better and offers more potential, it's a short stroke engine that works much better on high revs etc.
Yes, it's a lot more work involved to make the Duratec fit but the potential of the engine - considering the fact you're looking for power - is worth the costs and time you'd have to invest.