Terminating with Master's after several years in a PhD program. How to indicate on CV/Resume?
Personally, I would just put the M.S.  People might assume that you were working towards a Ph.D. and decided to leave with a Master's, or they might assume you were working on it part-time; if you think it is important to distinguish, then you can mention it in a cover letter.  But I would guess that, while dropping out of a Ph.D. program might not necessarily look bad, working towards a Ph.D. that you didn't complete isn't something you want to put on your CV.

(Disclaimer: I've never been in a position where I have evaluated people's CV for hiring, so this is just my guess).