That's fine, I was just pointing out that if you write it that way, the question naturally arises: shorter than WHAT? Longer than WHAT?
"Shorter" and "longer" imply a comparison
If you write: "The principal difference is the wavelength of the two types of EM radiation. The wavelength of visible light is longer than the wavelength of an X-ray."
EDIT: Or you could just omit the 4 words "which have shorter wavelengths" from the end of your answer.
Either way, that's a better wording because it makes it clear WHAT the comparison is. However, the answer is still vague. If you actually specified how many orders of magnitude longer the wavelength of visible light was, that would be even better:
The wavelength of visible light is 10^{blah} times longer than the wavelength of x-rays.
Even better would be to explicitly state the wavelengths. Physics is a quantitative science.