Hi, Andrew—Do you aspire to follow a specific code of style conventions, such as APA, MLA, or Chicago? If so, you should consult the respective style manual.
If not, either orthographical form is fine, as you have already discovered. I myself dislike the apostrophe there; however, if you like it, I can respect that.
If you want to avoid the issue, you can simply write "mid-nineteen-seventies," as the New Yorker does. I consider it the best-punctuated magazine in the U.S.