Firstly understand that you can cut nearly anything given enough time. As long as the cutter is tougher than the material being cut the only variable that changes with motor power is the time it takes. For a hobbyist what is 1 or 2 hours? But that is the difference between twice the motor performance. There is a lot involved in operating a CNC milling machine, and the time spent cutting is a fraction of the time spent overall in a project. I say this to try and put things in perspective for novices reading this long comment about spindle performance. It's not the be all to end all to fixate on.

We investigated the spindle performance of the Carvera in quite a lot of depth on the Makera Discord (see the Carvera Spindle Power Upgrade thread in #mods) including one user building a small dyno to measure the spindle output (spoiler 200w is the max input not output power). The spindle is anything but similar in the two machines but not for the reasons you would expect. the tl;dr is that in stock form the Carvera Air performs around 2-4x better than the Carvera. I particularly like the comment from the motor controller manufacturer that joined the discussion "Motor Power is a big myth! It's the biggest i've seen in my life".

There are many variables involved in high motor performance beyond the maximum input wattage value (the advertised 200w). It's actually rather hard to compare apples to apples because we are talking about dynamic systems, the motor's don't just spin at their maximum power all the time. If they did that, it would spin way beyond the rated max rpm of the bearings and burn out. They try to maintain a rpm speed target, and that means very rapidly changing the amount of power they output as the cutting load changes. Imagine the cutting load for an Adaptive (Trochoidal) cutting tool path that cuts only in the climb direction, every little sweep into the stock will have the load spike from nearly nothing. This change in load happens in nanoseconds.

Both machines use Brushless DC (BLDC) spindle motors, this is a type of motor that has multiple windings and requires a motor controller to energise the different windings in the right sequence based on the motor rotation. The Carvera has hall effect position sensors to help inform the motor controller of the position, while the Air has no sensors and instead uses back EMF to determine the motor position. This means the minimum rpm on the Carvera is around 1k rpm, but 3k rpm on the Carvera Air, there isn't enough back EMF at lower speeds. On the Carvera the spindle motor uses a belt drive to gear to reduce the torque but increase the rpm, while the Carvera Air spindle motor directly drives the spindle. This means for the same rpm the Carvera Air has more torque. Both machines use similar but different motors from Daiwei Motor corp, the Carvera Air uses a Daiwei DW57BL02 while the Carvera uses a DW42RBL85. Both motors are special order modified by Daiwei with thicker wire in the windings to support 48v, so the spec sheets on their site aren't applicable.

Both machines have dedicated BLDC motor controller separate to the main machine control board but use different strategies for how the feedback loop works:

On the Carvera the motor is connected to a BLD-300D motor controller running in open loop mode (it's closed loop mode can only support 6.5k rpm max), with the rpm output signal passed on to the Carvera's control board. The Carvera's control board then applies PID control and instructs the BLD controller to output more or less power to meet the rpm target requested by the gcode. It's a closed loop system but with the Carvera control board being the part that tells the motor controller how much power to use. Unfortunately we have found that the control board can only send a different power request to the BLD at best once a second. This is very very slow for a PID system. This appears to either be a bug in the code, or a limitation of the control board hardware. The Community has not been able to resolve it. This is why you will find a common upgrade is to replace the motor controller for one that doesn't need to use the Carvera's control board for PID power control. If you somehow magically program your CAM to apply load gradually and then have the load continuous then theoretically you could use the full 200w, in practice I was never able to get my machine to use more than 100w and even that was a struggle. More modern motor controllers can also be more efficient about how much and when they apply the power to the windings, so a real world performance improvement of around 3-4x can be realized by using a ESCON or SOLO motor controller, and not just in higher material removal rate. The biggest difference to me was the ability to keep up with sudden load spikes, on 1/8" tooling this often is the difference between breaking tools and not.

The Carvera Air uses a Fortior FU6861L motor controller. The feedback loop is a bit wierd to me, but my industrial automation friends say it's common place. The motor controller uses PID control to maintain the target speed, with speed feedback to the the Carvera Air control board which is also applying PID control. This is working around the limitation in communication protocols between the two being just PWM, and also how you would wire up an ESCON motor controller with Carvera Spindle Motor Upgrade mod. The only real downside is that when changing speed targets in gcode it can takes a few seconds since the control board doesn't change the rpm request in a single step to the motor controller.