A fair point Harrison...
I should really just be saying that there are a number of indications suggesting that six degrees of movement are superior to those with less degrees. I won't search for the pubmed articles now, but they exist.
However there are also plenty of stories of people with a elastomer based disc that had poor outcomes while you've had a great outcome with your older device.
At the same time, there's a reason the Charite is no longer being implanted.
(And I'd probably choose a ProDisc over an M6 even though the M6 offers 6 degrees... based on some of the M6 failure stories and images... Especially multi-level.)