For me I would say do it as a part of your Advanced Open Water course. Many locations will allow you to do it as an add-on to AOW with a lower course cost as a result. For example here at Silent World you can add 1/2 day of class time to your AOW to do your Nitrox and get it for $79.99 plus course materials.
Any time you are doing double dips on deeper sites Nitrox becomes almost needed to not hit your NDL when your doing the second of the two dives.
Any time you are doing double dips on deeper sites Nitrox becomes almost needed to not hit your NDL when your doing the second of the two dives.