Difficult. I think that its hard to compare because of the different methods of access and expectations. Is the NHS at the cutting edge of medicine? Not always. My late husband was considered for treatment privately that was not yet available on the NHS at the time and is now hedged around with triage and caveats. Sadly the treatment would not have been appropriate for him, but it wasn't even considered on the NHS.
Aricept and the other Alzheimers drugs are an excellent example of an intervention that was (might still be) more easily accessed in the US by reason of funding.
Being at the leading edge isn't always a good thing. In the US it was standard practice to give EPO (erythropoetin) to boost production or red blood cells as an adjunct to radiotherapy for cancer, particularly breast cancer. Its now being discovered that EPO can stimulate cancer cell regrowth. The Uk is benefitting (sadly) from that US discovery.
From what i hear from my US friends, if you are insured, there are more options offered in the US.
Speaking as a a retired NHS employee and someone who has had recent experience of both NHS and private treatment for my DH, I am not giving up my private health insurance!