Graham,
Difficult to compare preformance without motor curves at different voltages and amperages . So one would need to see what torque can be had at 3 amps for each motor.
Maybe you will only get half the torque out of the 566 in comparison to the 263 which may have full torque. Additionaly depending on how they are wired that torque value
will change. The max speed of the motors are voltage dependant and also influenced by the motor characteristics. As speed is increased torque will decrease and that also varies
depending on how wired. In general the higher the torque rating the slower the max speed. What you are are interested in is power which is max torque over some speed range.
So the motor power range will vary depnding on voltage and amperage available and you are limited by the driver and possibly the power supply also. So in general
one would need four different motor curves showing preformance, two for each motor which shows different curves depending on how wired. You need to use curves specific to
the motors and not compare to other manufactures information.
The general statement "in series to give you better torque but slower speed or in parallel lower power but higher speed" are true, but, one needs to consider / compare
more than that when comparing different motors for an amplication since other things come into play.
RICH