An MRes is a training in research methods and approaches. It will give you an advantage if you want to move into research or study for a PhD. Whilst your potential employer / PhD supervisor may not differentiate between an MSc and an MRes, if the course was any good you should find that you have a far better understanding of research and whether it is for you. This will improve your performance at interview and in the early months of your new career/PhD. If you're not moving into research then you'll probably find your potential new employer has no idea what the difference between the two degrees is, but will probably think the 'research' bit means your clever.