No, steel is not better in your case, it's just different.
Open steel (or titanium) design is better for long term use. I am always caged, so it makes sense to go metal. It's easier on the skin, easier to clean, easier to design it with more space and air. From your post I understand you will stay locked when she is out. That's hardly long term use, unless we speak about your wife going on a vacation with the bull.
Some of the plastic cages have kind of an open design - Kink3d Cobra for example.
The most comfortable cage I ever had was plastic HolyTrainer v5 nano - completely closed design, but not the greatest for long term use. In your case you don't even need the most comfortable cage, because you will probably not slееp in it.
If I was you I would just order the first cheap cage I find on amazon or lockthecock or houseofdenial, it will probably be the wrong size anyway. Doesn't matter is it metal or plastic. If you decide to go more into chastity and stay caged for longer, then you will know better what you like and want and also will know more about your ring size. The tube size is easy - your smallest size when flaccid, that's it. But the ring - you will have to try.
Good luck.