hi,
to be honest, i wouldn't have thought that a potential PhD supervisor would care whether you had an MRes or an MSc, providing you got a good grade and it was relevant to the PhD you want to do.
obviously the MRes has a heavier research element (in a lot of cases an MSc spans 1 academic year, ie. Sept-July, whereas an MRes often spans 1 entire year). dependent on your subject, the MRes is generally 6 months taught + a 6-month research project, whereas the MSc tends to be more taught with a small project at the end. therefore, for the student, it may be better to take the MRes as it would be better preparation for the PhD in terms of managing your own project, coming up with your own ideas, working independently etc.
having said that, there is certainly nothing wrong with doing an MSc, it would be just as valuable, and i think the main criteria for choosing which to do should definitely be which is most relevant to the research you want to do in the future.
hope this helps, and good luck!!
ps. it might be worth noting that i am slightly biased as i did an MRes instead of an MSc!!!