Personal preference at the end of the day.....the list of pros and cons is endless.
Factors to consider......your budget....either go all out or get a rolling project....
You say a sierra cosworth, if so thats a 3dr and 2wd unless modified to 4wd.
Sierra sapphire cosworth....untold options available but again its your budget that will ultimately decide what route you go.
I always set out to get a 4x4 sapphire as i wanted a later model car.