A couple of considerations:
Is there someone in town who runs the set that you can ask for advice?
Will the hobby shop let you read the instruction manuals so you can see if they're understandable?
What extra bits are available -- extra power modules, circuit breakers -- and how are things hooked up?
Is one hand controller easier to operate than the other? Do you have to do extra key strokes for some functins?
What other units (brands) are they compatible with?