I've had both and there is no doubt the ST is a much firmer ride. Having said that if you are going to get the best out of it on country roads it has to be so it handles like a dream. It's a compromise but only you can decide if it's one you can live with.

See my parts for sale on thread "philf1s Spirit Blue Fiesta ST180 parts for sale"

Will you notice it on a short test drive as I don't want to commit and then hate the thing a few months down the line 

^ I'd see if you can get a 24 hour test drive, I know some dealerships can do this. If not it needs to be a good long drive on different roads. If there's a dealership near where you'll do most of your driving, see if they can get a ST for you to test drive and then you can take it on the roads you'll actually be driving it on.

Good plan 

My ZS with the 17's and low profile tyres doesnt exactly feel major plush comfort wise tbh on certain roads 

From someone that's owned a a mk6 Zetec s, mk7 Zetec s and a mk7.5 Zetec s I really don't regret getting an ST. The ride is firmer but ive never felt it back breaking. I know some cars if you hit a bump it literally feels like the car has no suspension at all (such as type r ep3's). The seats I find so much more comfy than the Zetec s seats and hold you in much better on bends as well.

Try and test drive one, and can decide from there? As other ST owners have said you will love it! Had 50mpg out of mine on a journey to :)

Decided against the ST (I think lol) and I'm going to keep my Zetec S, Superchip it so it feels like a new animal and hold onto it until the new shape Fiesta is launched as that's rumoured to be in 2017 

Just need to keep an eye out now on the progression of the new generation Fiesta