Are there differences between SM, MS, MSc, MSci degrees?
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.