If your article or story has not been published, I recommend you sell first rights, rather than simultaneous rights. Many editors will not show interest if you are shot gunning your material to a number of publications at once when the piece has never been published. Plus, you will usually be paid more for first rights.
However, once your article is published for the first time, by all means feel free to sell reprint rights on it. You will probably earn a third to a half as much for reprint rights. Nevertheless reprint rights are an excellent way to earn extra money by selling your manuscripts over and over.
“One-time rights” give a publisher the opportunity to print your material one time. Use this terminology when selling a piece for a book compilation since books take a long time to come out in print. In the meantime, you can resell reprint rights on the piece. Also, one-time rights may be confusing to the editor, who may wonder whether or not your material has been published before. As a result you may be paid a lower amount than first rights would be given. Also, you can offer one-time rights to publications in other countries, particularly in the Third World, on material for which I own the copyright.