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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> 
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and

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> The differences between these concepts are pretty complicated.
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Answer: 
*Pretty complicated* is approximately the same as *fairly complicated*: there is a significant degree of complication. It is complicated enough that it will require much effort for an ordinary person to understand it.

* *Pretty*, in this sense, is used mostly in conversation, very little in formal discourse.

*Quite complicated* is more complicated than that: *very complicated*. It is so complicated that an ordinary person may not be able to understand it entirely. 

* *Quite* is used mostly in formal discourse, much less in ordinary conversation.
one would use pretty in this context so that the speaker liked to talk about the mentioned concepts

i would use quite in this context maybe to hint that the speaker himself would rather not talk about the differences between the concepts in full detail -- which is absolutely understandable, as they apparently a r e "quite complicated".. :-)

hope you accomplish to get message across . good luck