158s and 156s are best suited for the circles. Turbos are no good for stop start journeys. They are designed for long distance runs at higher constant speeds. 100mph units that can get up to 90 max once or twice during their journey round the circle.
Fair enough but the doors at the end of the coaches on the 156/158's are a nightmare when the train has a standing load which is a very regular occurrence.
The 170 is better as it is easier to get on and off, particularly for a stopping service. The 170 is also generally a much more pleasant experience.