Here are my two, won't go into detail as i get asked 10 times a day to compare them.

As mentioned already the RS is a very raw car. It feels like its an RS. The ST is basically a comfortable Ford that has had a 2.5turbo added. I can throw the RS into corners that the ST cannot handle and understeers. The ST is beautiful to drive but when someone starts playing i wish i had the RS out. The sound of the 5 pot in the ST is great.

All in all two very different cars!!