Ahh... the old "compare and contrast" question.
Simple engineering answer: KL4030 will handle slightly higher voltage. Higher voltage gives you more RPM. Depending on the drive ratios and motors you MAY be able to get more RPM (IPM). OTOH a smaller motor with a wider torque-RPM curve may do the same thing. Remember that steppers lose torque with RPM. Bigger motors typically lose it faster than smaller ones.
The Xylotex cards are built in the US and the tech support is better. The manual is definitely better.
The Xylotex 3 or 4 axis card is much smaller than 3 of the KL4030's.
We have sold over 50 Xylotex based stepper control boxes. Only one has been returned and it was repaired by Jeff for free.
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Stepper & Servo Systems