So...having said all that, let's look at both.
Opalite is sometimes called Opalized "Quartz", Sea Opal, and Opalized Glass. It is a manmade glass that is very pretty but often confused as a gemstone. Some vendors do it on purpose while others just aren't educated. I once had a visitor to my booth at a show looking at an Opalite necklace and commenting on how inexpensive it was. She said that she had bought one recently for several hundred dollars and she just loved her "opal" necklace. I was sorry to inform her that it was a manmade glass rather than a gemstone. She was very disappointed. There's nothing wrong with using Opalite in jewelry but designers need to be fully educated and honest with their customers.
I really enjoy the properties of Opalite - it picks up the colors, or changes color, based on the colors surrounding it. On a black background it will appear more blue, while on a white background it glows more orange/pink.
Rainbow Moonstone on the other hand can be supremely beautiful and expensive when found in high quality pieces. It can also be referred to as Ceylon Opal.
"High-quality rainbow moonstone is usually chatoyant and sometimes displays a strong cat's eye. The floating-light phenomenon and sheen of moonstone are called adularescence or schiller."
From the Rings & Things Gemstone Index
Using rainbow moonstone in my work is a challenge - mainly because it can be difficult to photograph the beautiful blue flashes present in high quality stones.
Rainbow moonstone goes with anything - any color and any season. It's a versatile stone that plays well with gold and silver. Pairing it with other stones is fairly easy because nearly everything matches. I am a great fan of this stone.
It also comes in peach and gray moonstone but I don't like those as much. It's funny that some vendors are calling it "silver" moonstone. I suppose that does sound a bit better than gray.
Rings and Things