I need to update my write-up on this topic, but the bottom line is that the 12mm is superb on the M9, especially with the latest v1.162 of the M9 firmware. Just code the lens as the 21mm f/2.8 Elmarit-M pre-ASPH (11134). The correction is perfect, no red edges, no cyan drift, and excellent vignetting correction.
I do prefer the LTM version for a couple of reasons. First, it's much smaller--much, much smaller. Second, it focuses much closer--down to 0.3m. Yes, its scale focus, but at that FL, you pretty much have infinite DOF if you stop it down even a little, so it's not an issue. Finally, after trying several copies, I've consistently found that the LTM version is just a bit sharper overall. Theoretically the optical formulas are the same, but something about the LTM version is better.
That said, the coupled version is nice because, well, it's coupled. It can also take a filter if you feel the need to use one.
The new finder is a huge improvement in quality and is much smaller. I use the new finder with the LTM lens. Best combination out there.
Jeff