I dont have any of these cubes, but I have tried them both at comps, and I prefer the little magic, but not by much at all. The little magic is like 8.75/10, and the MF3RS2 is 8.5/10. (IMO)
MF3RS2. Love the feel after lubed and tensioned properly. Still fast and stable too. Back up main as main is GuoGuan YueXiao (not pro, I know, I'm weird).
I really like the insane speed of the Yuxin Little Magic, and the satisfying smoothness of MF3RS2. So, knowing that springs affect the speed of a cube to a certain degree, I fitted my MF3RS2 with my YLM springs.
The result was better than expected; my MF3RS2 is now so much faster, while retaining its smoothness. However, it is a bit unstable, but how much can you ask of a budget cube?
Note: I don't recommend trying this, as I am not sure if there are any bad side effects from using different cube components on another cube. I hope to find out if this test is successful once I break it in after a few hundred solves.