Either one should work I beleive, but if it lowers the stance of the car I would go 18" rims. Same setup and 19" rims would be a hell of a look on a SVT.
For one you have increased the footprint with the wider tire 205 (16",17" rims) compared to a 215 or 225 tire (18",19" rims). Plus with a more sport type suspension from Bat Inc., it sould handle like a Porsche 911 on the Autobaun.
Remember more rubber on the road the more control you will have plus better traction to play with or burn!!
