Well, first I must say that
I am not qualified to tell anybody what disc to use. There are others on this forum that have probably read much more than I about the subject. But here are some things
I found, which I have used to make up my mind:
Manufacturer of M6 giving the benefits compared to some other discs:
http://www.spinalkinetics.com/wp-content/themes/sk/resources/7-Competition.pdf
Dr. Bertagnoli raising much criticism against elastomer discs:
http://www.dr-bertagnoli.com/artificial-disc-replacement-center-of-rotation.html
Correct me if I am wrong, but Bertagnoli does not use elastomer discs?
Although, I am not very impressed by prodisc-L either. It has been proven that it loads the facet joints significantly more than movable core discs. This has been shown both in theory:
Comparison of Biomechanical Function at Ideal and Varied Surgical Placement for Two Lumbar Artificial Disc Implant Designs: Mobile-core vs. Fixed-core
https://www.researchgate.net/publication/6078599_Comparison_of_Biomechanical_Function_at_Id eal_and_Varied_Surgical_Placement_for_Two_Lumbar_A rtificial_Disc_Implant_Designs_Mobile-core_vs_Fixed-core
"Placing a mobile-core TDR design anywhere within the disc space reduces facet loading by > 50%, while the fixed-core TDR design increases facet loading by >10% when compared with the intact disc in axial rotation, the worst loading case."
and in vivo:
Effects of lumbar artificial disc design on intervertebral mobility:
in vivo comparison between mobile-core and fixed-core
http://europepmc.org/abstract/MED/21153595
"A prosthesis with a fixed-core increased the VT during flexion–extension motion in comparison to the physiology and mobile-core prosthesis at both L4L5 and L5S1 levels."
What do you think about these references?
If anything is certain, it is that there is no such thing as a perfect artificial disc, one just has to weigh the options and take the "least worst one". 