hi,
It's funny you should ask because I was in NYC today looking at the Xprong by Vatche and I also stopped by Tiffany's because I had to get my wedding bands polished and stopped by the 2nd floor to look at the Lucida rings.
As far as I could see, the Lucida and the X prong are exactly alike, including the thickness of the prongs and the band itself.
The only difference is that you can only buy a "Lucida" cut diamond from Tiffany's.
In fact, when I asked the salesperson if they made the setting with round diamonds, she sarcastically said "NO, and if you do find a lucida setting with a round diamond, it is a knockoff. We have a patent on this cut and design". You would think she owned Tiffany's or something!
I wanted to tell her that Vatche manufactures their Lucida settings, the x prong is not a copy, it is the same exact thing, made by the same exact people. So there are many people with the Lucida setting and a round diamond, and a princess cut, and a radiant,etc., etc., and they do have the REAL thing. But why waste my time.
So basically, there really is no difference. Oh, except for price. Just to let you know, I asked for the price of a 1.51 Lucida in platinum H VS1 and it was over $20,000.
Another thing, their Lucida's come cut as princess and as radiant cuts (square or rectangular), but I couldn't tell if it was cut cornered. It doesn't really matter though, because the prongs cover it anyway.
You are better off buying a nice princess or radiant cut and getting the x prong and saving $10,000. Then, for 5 bucks, you can have Tiffany&Co PT950 engraved in the band.
And to answer your last question, the x prong can fit many different shaped diamonds, it's just a personal preference.
Good luck!!!!