Okay, so with a little homework, I realize I was very mistaken about this movement. I'm not sure what I was confusing it for. At first blush, I was thinking of a simple, vanilla JLC ebauche used by AP. Only a fraction of a fraction of that is true. While AP, Patek, and VC did commission JLC to develop the movement, it is nonetheless technically quite remarkable.
That said, the original point I was trying to make (though overstated in the details) remains true: the RO is most distinguished by its case and bracelet design (as the 920-based 2121 is quite prolific), and the Cartier's Piaget's movement does deserve respect from watch geeks.
No worries Foo. The both movements are very respected, top quality, and either will serve you well.
The RO for me could be a 1 watch for everything, if I had to be a 1 watch kind of guy. The Tank is one of my favorite dress watches. It can be dressed down, but for my lifestyle, the RO gets more wear.
As for the actual watch cases I like each for different reasons. I find the Tank is almost a study in minimalism and elegance. Its clean lines and simplicity are part of what I find so beautiful and appealing. The RO, is all about complex angles, mixing textures (brushed, polished, and petite Clou de Paris), and tremendous workmanship. As stated before the longer I've owned my RO the more I have come to notice interesting nuances such as how the light plays off the various textures, how labor intensive it is to make, and how the finish exceeds that of almost any other brand (I found only the PP Nautilus to be of the same quality of finish, I own VCs...but did not think their Overseas was up to the same standards as the RO or Nautilus).
In the end, I'm equally biased as I have a CPCP Cartier Tank XL and a RO 15202...and the best advice I can give you is get BOTH !!!