which one's better?

S-Class. The DeVille is more of an E-Class competitor. I found some pics of the Cadillac ULS (Ultra Luxury Sedan) on Google. That would be in the range that of the S500, 745Li, A8, LS430, XJ8, etc.

The only other top-of-the-line vehicle it might compete against, would be the Acura RL in price, but not in performance.

If Cadillac just moved it more upscale, then they might be in more with the other luxury brands.