Hi, I'll chime in as I've recently bought the new Memotron M2K after much research.
First, both the Mellotron and Manikin sites are disasters! I had to email both Markus at Mellotron and Thorsten and Manikin to find out info that wasn't available on their sites. Thorsten answered me right away. It was only through scouring forums and Facebook groups I got all the info I needed.
I play in a NJ based Genesis tribute band covering the Seconds Out tour, and have been using M-Tron Pro with the Streetly expansions with a Receptor. We did our first show, 600+ sold out!, in April. I'm using a real RMI piano and ARP Pro Soloist, and the rest was plugins. Let's just say depending on vintage gear AND plugins on stage is scary! I was looking for more hardware to ease the load of the Receptor.
Our soundman is a Clair Bros. vet of Yes tours, plus he saw all the US Genesis tours of the 70s, and he's VERY critical of digital emulations. We decided I must have more flexibility and variety in regards to my Mellotron sounds.
I chose the M2K for its library of the most popular sounds. Having around 10 of the famous "3 Violins" sound is more vital than I would have thought. Now, in rehearsal, I can easily swap between Streetly's M400, Tangerine Dream's M400, King Crimson's MKII, and others to find the best EQ'ed version for each song. There are also quite a few versions of the "8 Choir" tapes.
Another draw of the M2K is the onboard effects. I need a big reverb on these Mellotron sounds that I don't need on the RMI, and my ARPs need tape delay.
Lastly, the M2K's FRAME options are great in a live setting. Take Genesis' "One For The Vine". All of my MIDI controllers are in use for this song. Now, with the M2K, I have the A/B/C tapes set to play always as a layer instead of crossfading across the dial. I have a 3 Violins in its original place across the keyboard. I can do those suspended chords Tony does at the beginning in the lowest octave. Then, when the big chords come in where Tony is playing two-handed on the record, I have a layer in the upper octave. Then, when the overdubbed "ah's" come in, I have the Fairlight's ARHH as a high velocity layer. I can do this all with my left hand while I play the RH piano part on Ivory with my Nord.
That said, there is very little info out there on the "new" M2K. The OS is the same as the M2D, with the FRAME/PLACE settings and the three levels of FX. It also has lost the CD drive and loads instantly. They do not have this on their site, incredibly!
EDIT: I should add that the build quality of the M2K is fantastic. It's lightweight to carry, but sits solidly without wiggling. It sits atop my RMI piano nicely without sliding around like my Novation Impulse 61 did previously. Also, the new M2K has 37 keys unlike the previous version. The M4000D non-Mini looks great with its wooden keys and XLR outputs, but with the new M2K at the same price point as the M4000D Mini, the M2K had more options I needed for this project: the many versions of the classic sounds, the effects, and the option to transpose and layer. If I wasn't in this band, I would have saved for a Streetly M4000!