JimL is right on, and even more so, the certification rigor for a Part 27 aircraft (Normal Catagory Rotorcraft) is much lower than that for a Part 29 (Airline Transport Rotorcraft). Fire protection, crash safety, duplication of systems, etc all favor the Part 29 aircraft.
The difference is like that of a Beechcraft to a Boeing, literally.
The 427 is the Normal catagory aircraft, and is quite a bit less than a Part 29 aircraft, especially regarding safety.
The EC 135 is a hybrid, with its twin engine installation the only system meeting Part 29
The 155 is purely a Part 29 aircraft.
Comparing the three is not easy, they are an apple, a tangerine (hybrid) and an orange.