I may be able to shed some light here...I'm a scientist doing a PhD. In my group there are two medics, one doing a PhD and one doing an MD.
The person doing the MD is running a clinical trial for his degree which involves lots of people working under him and lots of patients across the UK. The person doing a PhD (and I) has to run all his own experiments and basically do all his work by himself. The scale of the project is hence much smaller. In his thesis he also has to very clearly define what he's done and possibly can't include any work not done by him personally.
Both degrees require you to make a novel contribution to knowledge which makes them similar in terms of academic rigour. Neither is more or less work than the other. The kind of work required is different. The MS requires more management and the PhD is more hands on. Both require the same amount of critical thinking, experimental design, analysis etc. In terms of writing up though the PhD word limit is 100,000 words and the MD word limit is 50,000 words for my uni (may differ between unis).
The MD pathway is only available to qualified medics whereas the PhD pathway is open to everyone. So I suppose the choice for a medic depends on what kind of research they'd like to do. The person doing the MD is now a consultant and will continue to be involved in our future clinical trials. The person doing the PhD works at our Prof's clinical at the hospital but spends most of his time here teaching and doing basic science research.