For me, it depends on what car I am testing.
A rather low powered car would be tested at a smaller, somewhat less complex course.
I really don't see the need to test such car at Nurburgring where it will do every corner perfectly due to how slow it goes.
Examples: Madrid, Tokyo, Daytona Road Course (To some degree), Tsukuba, Cape Ring (Less complex variants), Eiger, and Grand Valley
Higher powered cars are more suited to more complex courses, along with some straights.
A more complex course really tests the car, it's accelerating and corner capabilities, and such.
Examples: Willow Springs, SSR5, Cote d'Azur, Matterhorn, Nurburgring, Grand Valley, Monza, Circuit de Spa-Francorchamps, and Twin Ring Motegi.
Unless I'm testing for a specific course, I'd generally go for the above courses.
However, I generally don't go for Sierra. Sierra in my opinion is too complex and too long. To get back useful information on your tests would mean doing laps that would take several minutes, and doing multiples laps would take a long time.
I don't recommend using Sierra for testing.