Hi Chris
Depends what you're going to do with them. If you only plan to use your own cylinders rarely then you probably one one AL63 and one AL80. They are cheap and are what you will encounter most frequently when the dive operator provides cylinders.
If you will be diving your own cylinders regularly, bringing them with you on most dives even when rental cylinders are available, HP steels are a good choice. I would suggest getting an HP80 (or two) for your wife. It will be shorter and lighter than an AL80. Then you could get an HP100 (or two) for yourself. It will be roughly the same size and weight as an AL80. I would think you would want to start with two per person.
You could conceivably dive an HP120 but the extra air will not do you much good, since with proper gas planning your dives will be limited by the gas your wife is carrying.