I have owned them both as well - they're both fantastic IMO. Both are excellent on the low-end - I think the Sapphire is even a bit better than the C4 - but I listen to mostly Rock, Blues and Jazz, and not to solo piano, so I'm not sure that I would disagree with Bar81's comments.
I think the Sapphire drivers are somewhat better integrated - just a very coherent speaker. The C4's need to be listened to from a further distance. The C4's also delineate the individual components of the music more distinctly - whether this is a good thing or a bad thing is a matter of taste, but listening to a five-piece rock band on the C4's gives you more of a sense of the five individual players.
The C4's incorporate Dyn's DDC, which helps control radiation patterns and makes this very large speaker work well in less than perfect, huge rooms. The Sapphires do not incorporate this, but are more room friendly to medium-sized rooms.
I think the C4 is a fantastic looking speaker, especially given its size. The Sapphire is a work of art, one of the few components of any ilk that made me want it strongly due to its aesthetics.
I love them both, but would say that they do have distinct differences that could make one preferable to another. And, as Dhaskins says, they are apparently getting close to being sold out. No idea if they'll suddenly decide to do another non-anniversary run of them, or if they'll just do the natural show-up-used thing so many pieces of gear do, but the Sapphires are definitely a special speaker if you're a Dynaudio fan.