In general the S1 and S2 shocks and springs are both the same. They will fit mk3 or mk4 cars, inc the same (mk3 Escort) shape orion either way.
The standard S1 suspension was harder and more motorsport based whereas the S2 was manufactured softer to appeal to the up & coming yuppie generation who couldn't afford Porsche cars but wanted something quick and more affordable but still daily drive-able, so had a less aggressive and noticable bodykit & spoiler and softer suspension for town driving.

After market upgrades now make little difference as the kits will fit and work on both cars. You will also find it difficult to buy new original suspension for both cars and if you find some it will be quite expensive, especially compared to uprated equipment. The concours and S1 purists will definately want the original parts so they become more expensive due to lack of new stock.