There really is a big difference between the various helicopters from a design standpoint, mostly driven by age, since the original design more or less reflects the technology available at the design date, and the rules and requirements in effect at the time.
Many key safety improvements have been instituted in the last two decades, making a big change in the ability of the design to withstand damage or failures, and therefore a big change in its overall safety.
A "new" helicopter need not be a new design, it must only meet the standards in effect when it was designed, a principle caled "grandfathering". Without grandfathering, all manufacturers would have to change their designs and re-certify them each time a rule was changed.
Regarding the FAA/CAA/JAR regulations in effect when they were designed, the S-61 was designed in 1960, the Puma family in 1974, the Super Puma in 1978, the EH-101 in 1990, and the S-92 in 2000.
There are very considerable differences between the designs of those eras, just as you can see the evolution of cars in that time period. For cars, crashworthiness, anti-lock brakes, side panel structure, interior safety, tire standards, etc all reflect the better understanding of safety engineering.
In the late 1990's the helo regs took a big jump, and required many new structural and fatigue improvements that were determined based on transport helicopter accidents. Things like Structural Flaw Tolerance, Bird Strike, Engine Turbine Burst protection, Crashworthy Fuel were all introduced between 1991 and 2000.
You can buy a brand new old design helicopter if you don't know the ins and outs!
see:
<a href="http://www.airweb.faa.gov/Regulatory_and_Guidance_library/rgMakeModel.nsf/CurModel?OpenFrameSet" target="_blank">http://www.airweb.faa.gov/Regulatory_and_Guidance_library/rgMakeModel.nsf/CurModel?OpenFrameSet</a>
for a place where the Type Data Sheet for the model lists the FAR amendment and date of that amendment, which effectively dates the level of technology of the design.
See:. .<a href="http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/MainFrame?OpenFrameSet" target="_blank">http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/MainFrame?OpenFrameSet</a>
for the historical FAR which allows the real geek to look up individual FAR paragraphs to see the growth of the safety requirements, such as Engine Turbine Burst.