The RPM rating of the motor does not affect the quality of the motor. The 2600 RPM motor is designed to run at 2600 RPM under full load, and the 2800 RPM motor is designed to run at 2800 RPM under load. One of these motors might be a better choice over the other depending on the scooter's sprocket gear ratio and the type of terrain that the scooter will be used on.
The bolts on the back of the motor in the first image are only there to hold the motor together, however the bolts on the back of the motor in the second image can be used to adjust the brush timing of the motor. If the motor's timing is more advanced the motor will produce maximum power at higher RPM, and if the motor's timing is less advanced the motor will produce maximum power at lower RPM. The best brush timing setting will produce the most power at the RPM that the scooter is most frequently used at.
Another thing to consider is that electric scooter motors which have the same nameplate ratings but are made by different companies often have different performance characteristics. For example a 36 Volt 800 Watt motor made by one company may not be as powerful as a 36 Volt 800 Watt motor made by another company. So if you have both motors then testing both of them on the same scooter may help to find the most powerful motor out of the two of them.