I've made presets with both. While they share a lot, they are very different.
Synthmaster is a very, very powerful synth with a billion options. It has analog, wave scanning and vector synthesis, phase modulation, additive synthesis, sample playback, and FM. It is called Synthmaster because it is one synth that can do the kinds of things many synths do. For this reason, very gifted programmers like Nori Obukata can make sounds on it that sound like other famous synthesizers throughout history. I don't think most Synthmaster owners program from scratch with Synthmaster--they just use the presets, and they sell a lot of them. In practice, the difference for most between the Player and the full synth is the full one is more tweakable.
A very popular package includes all the expansions they have at the point of sale. It doesn't include any expansions that come out later. There's a much more flourishing third party preset market for other synths.
Synthmaster One is the opposite. It is designed to be easy to make your own patches with, by limiting the options. The design philosophy is identical to Rob Papen's Go2 and Arturia's Pigments. It doesn't do all the stuff above, but it is fun to make patches with Synthmaster One, and, as you noted, it has features that are not in Synthmaster.
I have heard that some of these new features will be in Synthmaster Three. I believe that will be like the Massive X--a different synth.
I don't know why KV331 gives all their synths the same name. It's confusing.
I hope this helps.