I agree with making them a synonym, but I think word-choice should remain the dominant tag, especially since it is used almost five times more often than differences.
In response to Ataraxia's comment, word-choice also can apply to sets of words like phrases (i.e., a choice of words) - it even says so in the excerpt and has been used that way. (Or, I guess you can take the English site's approach and use a phrases tag for those questions.)
differences, on the other hand, seems really broad. For example, will this tag include differences between verb conjugations, or would the verbs and conjugations tags be more appropriate? Does the word "differences" appropriately convey meaningful information about the topic?
Also, word-choice is a top 10 tag on all the other language sites like English, French, German1, Spanish, Chinese, and Russian. I know we set our own rules, but it may be worthwhile to take into account some level of consistency.
1: German also has a differences tag, but most of the other sites do not.