This cube should probably not exist, but it does, so we're happy. Buy one.
The Rs3m2021 was the cube that brought Maglev to the cubing scene. It is, for all intents and purposes, an Rs3m2020 with very slightly different spring compression settings.
The pieces are identical, same plastic, same mold, same shape or weight. You can interchange them flawlessly with the Rs3m2020. The friction is identical (the total friction of the springs is so minimal compared to the friction of all the pieces themselves that if you can "feel a difference" you're lying to yourself). The "springs" are... a bit looser: the Maglev lets you range on a slightly looser curve than the springs would, but mostly it's indistinguishable. If I didn't hear the Maglev squeak from time to time, I wouldn't be able to tell if it's the 2020 or 21 version.
As for its performance, it's a RS3M, it is fast, blocky, corner-cuts like magic, never (or almost) locks up and feels like a solid flagship cube. If the feeling was that the RS3M was too good to be as inexpensive as it was, the attempt to raise its price with this new iteration (a 40% increase) still leave it ridiculously cheap for the sheer quality of solving that it brings. If you're going to spend less than 20$ on a cube, this is the only cube you should really consider buying.
* Yes, maglev squeaks for some weird reason: did Moyu want to keep the signature spring squeaks from its predecessor?