Brackets Sprint 36 (which is within the next few days, as I write this), includes a new preferences manager. There's a section about this on the wiki. You should be able to do something like this:
var PreferencesManager = brackets.getModule("preferences/PreferencesManager");
// Whenever you need the current value
PreferencesManager.get("spaceUnits");
// If you need to know when the value might have changed:
PreferencesManager.getPreference("spaceUnits").on("change", function () {
var newValue = PreferencesManager.get("spaceUnits");
});