Are there differences between SM, MS, MSc, MSci degrees? I have a Master of Science degree from the EECS department at MIT. From the website it is pretty clear they call it an SM degree. I think many US universities refer to a Master of Science degree as an MS degree. My UK department has offered for a long time a program that leads to a Master of Science degree that we call the MSc program, but next year we are introducing an MSci degree. Do the different abbreviations officially mean something? Is it dishonest to refer to my SM as an MSc in the UK? If not, is it helpful?
There is no difference: M.S., M.Sc., and S.M. all mean Master of Science.  The difference for S.M. is that it is in Latin: scientiae magister.  

I have no idea whether it will actually help anybody's confusion to translate back to MS, but there is certainly no question of honesty.  For anybody with a Ph.D., however, I expect it will not make the least shred of difference, as a Ph.D. supersedes it quite effectively.