I have both a 1.6 and a 2.0 mk1 STW Foci at the moment, and to be honest, I'm a fan of the 1.6 engine. The 1.8 wouldn't have the benefits of either.
The 1.6 isn't the most powerful engine in the world, but you gain very little with the 1.8. The 1.6 on the other other hand has a better mileage, it's much more compact, much ligther, quieter, etc...