Hey Arthur,
I think you can’t tell this in general. All factors have an impact on the stiffness of the machine.
Also there are much more factors. If your bit is dull or you choose a too long bit which will bend, the best machine cannot compensate this. There is good knowledge and much experience required to make the best out of a machine.
If you ask regarding the Onefinity machines, the first thing I would recommend is not to use a hand trim router as milling motor. A strong induction motor as milling motor allows what a hand trim router would not allow: High speed at high torque, and a wide speed range with high torque to choose from. Machining wood requires both, high torque and high rotational speed. A hand trim router provides its nominal torque only at a specific speed, not over a speed range.
The Onefinity machines are extremely sturdy. They are monsters regarding weight. Their rails are made of 5–6 mm thick chrome-plated, hardened steel hollow shafts. Using a hand trim router on them is a like a bad joke.
Closed-loop steppers are often overestimated. In most cases, they don’t prevent a bit break and a blank damaged, even if the machine stops at a certain moment. Open-loop steppers are by far the most-used stepper motors for CNC machines. If they are correctly sized, they will perform as well as closed-loop steppers.
The drawback of stepper motors, both open and closed-loop, is their limited speed. Servo motors can be ten times faster, but their control electronics is more complicated. That is why they are usually much more expensive than stepper motors. Stepper motors have the steps implemented in hardware, servos only rely on the position sensing and controlling position by a lot of computation.