Question: What is the difference between *quite* and *pretty* in the following context:

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> The differences between these concepts are quite complicated.
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and

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> The differences between these concepts are pretty complicated.
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Answer: 
The usage of the word "quite" in modern English is a paradox. It can mean two opposite things:

The original meaning is still used sometimes:

"He is quite dead." (You cannot be slightly dead, so "quite" in this context means, "absolutely.")

The modern meaning can mean anything from "moderately" through to "surprisingly."

"They said I'd hate Javanese cooking but I found it quite tasty."

In this second context, the word, "quite tasty" could be substituted by the word "pretty tasty" and it would mean the same thing.
In many cases, it would be equally or more correct to omit either word and still retain the meaning.

"It is broken" instead of "It is pretty broken"

"It is sore" instead of "It is quite sore" etc etc