These two are really similar. Have you tried comparing them side by side? I often find that helps me a lot when trying to decide what to buy, especially with similar products. The "compare items" button is very useful.
I have the girdle and love it. The texture is awesome and it really feels good when used internally - I suspect it would also feel pretty good on the clitoris. Of course, that would depend upon how much your girlfriend likes texture.
Both products do use the same motor so the vrooms, etc. won't be different between them. It will mainly be a matter of size and texture.
I don't have either, but I'm interested in both, and if I had to choose I'd go with the conqueror. I like the girdle better visually, because the corseting is cute, but I'd feel that the conqueror would slightly more textural stimulation, since the detailing on the girdle is pretty small. I could be wrong though, but I don't see how you'd go wrong picking either one since they're so similar.