ccd and cue are almost the same thing, they're just like indexes to tell whatever program you're using how to load the img and sub. Both ccd and cue are provided to allow a wider range of programs to read the image. Personally I'd use ccd in this case, because ccd will use both img and sub, whilst cue will only use the img file. Though in most cases, it doesn't matter because the data in the sub file is unimportant and ignored by programs/games except for the very few rare cases where it's been altered for copy protection schemes. Daemon will probably let you pick between ccd and cue since it can read both.