You may as well sign up for all three major FF plans (AA, DL, UA), since there is no charge. But then you should decide a) which airline (or its partners) offers the best service on the routes you will be flying and b) what goals you have for use of your miles. In any case, it will be to your advantage to concentrate your flights on one airline (and its partners) as much as possible. If your goal is to redeem miles for future award travel, you should be aware that many travelers find it much more difficult to find award tickets at low rates on DL than on AA or UA.